After I pumped my tires full, and didn't even use the (new) bike for 1-2 weeks, one of the tire was not as hard to the touch any more. It's still quite full of air, but it bounces a little when squeezed, unlike the hard solid feel soon after pumping. As I said the bike was sitting inside for the week after pumping. The other wheel was better. What's normal?

I pump up before each ride or each day which ever is longer.

Road bike tires need to be pumped up before each ride.

It depends on how many holes they have in them!

Seriously though, this air loss is normal. If you want optimal tire performance, even from a 40 psi tire, you have to fill up every day or ride which ever is longer. If you are using really high pressure tires, 100psi or more, you can expect some air to disappear everyday.

Rubber is actually porous and lets the air though. A great example you can see of this happening a lot faster is a balloon. I think for people with money to burn, in exotic cars, they fill the tires with nitrogen or C02. The molecules are larger and don't go through the rubber as fast allowing for less top offs, but you have to buy the stuff as opposed to pumping free air.

My bike is brand new, though, only ridden 2-3 long distance, and total 5-6 times (some just nearby errands). The reason I didn't think it very normal is that the front wheel is very good, hardly lost any air, but the rear wheel has obvious loss. It's a 700 wheel. Probably the rear wheel came with more holes

The tires don't care if you ride them or not. The air leaks out at the same speed either way. Bigger mountain bike tires hold more air at less pressure so the pressure doesn't go down as fast as with skinny road tires. I find I need to pump my road tires up about once every 3 to 4 days for casual riding. The skinny mountain bike 1.25 tires about once a week. Big fat tires urban jump, cruiser or off road tires would be about the same once a week to maybe every 10 days. So it's very reasonable to expect your tires to be that low after that much time.

Rear tires get more flats, wear out faster as they have more weight on them compared to the front tire.

My front tires get more holes, from picking up thorns. The front ones blaze the trail. Every form of fauna here has thorns. If I see a stick or leaf in the road I avoid it. Off road I just cross my fingers.

Someone has to say it: As often as needed.

I pump mine once or twice a week.

Different in my case, though. The day I bought the bike, I took it by subway without riding, and I could already feel the difference between the two wheels. Maybe they didn't pump enough the first time, but the next time after both pumped, it's still the rear wheel that lost more air, so I think it's the wheel quality in this particular case.

Not the wheel but perhaps the tube has a slight weep through the valve or there's a very small pinhole manufacturing defect in the tube.

If that's the case, can't ask the manufacturer to replace it?

Replace the tube, its a $4 item. If you just got the bike then take it back to the shop maybe they installed it wrong or had a bad tube.

Not a defect, but if your bike shop likes to service whiners, go for it. Your next tube could have the same leakage rate...or not. If you have a schrader valve check to see if its core is tight; it just may be as bcrider says about a small leak otherwise. It's not a big deal and doesn't mean anything's defective. 1-2 weeks on my road bike I know I'm pumping up to my preferred pressure, mountain bike maybe not.

PS Get a gauge and be specific...

That slow a leak would not be seen as a defect. In fact I'm sorry I called it that. Tubes are made quickly for cheap. It stands to reason that some will be a shade better than the others and some a shade worse. If it leaked down to flat in a week then you'd have a case for asking for a new tube. But what you are seeing is perfectly acceptable and totally ordinary. If it's a schreader valves try venting some air out and then pump back up. Or even remove the core, blow it out and replace it. If there was a bit of grunge in the core seat this MAY slow down the weeping. Or not....

Learn how to patch a tube.

So do I, both my road bike and Mountain bike. Even when they feel find, I always find out I am at least twenty pounds low.

Many thanks, everyone! Yes comparing to some of your bikes, it seems not so bad. (Actually, if both wheels lost equal air, I wouldn't have been worrying )

mine generally needs 10 to 15 lbs each time before i ride. tire still feel hard but i always check

Agreed. If you don't use a gauge, you're just making wild guesses about the pressure in your tires.

If it really bugs you take it apart and fill up the sink and do a real leak test. If it's a cheap bike or a bike from a store that does high sales volume check the rim strips. An off center rim strip will cost you a tube real quick. My chinese cruiser had this problem. It had a hole in the tube three days after I aired them up for the first time.

Yes as other have mentioned daily if you want to keep it at a constant pressure. My weekend road bike gets pumped up before every ride.
My commuting bike I tend to do it once a week but pressure does drop as much as 15 psi in that time.

One time I got exceptionally lazy and I didn't fill my tires for over a month, and I ride 3+ times a week over south bay area roads. They were down 25 psi when I finally dragged out the pump and checked. No flats either. For some reason these tubes are awesome, and it takes near a month before I start noticing any difference in the quality of the ride/whine of the tires against the road at speed.

However, if you like to run a little low to gain some more plushness in your ride, you should check far more often than I do. I like my tires at 110 or so psi, both, and I weigh 160 lbs. I run a little higher by preference.

Buy a tire gage, for bike tires, and learn what the proper inflation pressure should be, and keep it there..

That's what I'm thinking after reading the replies here. How to know the proper pressure of my bike? Is it by feel or it's specified by the wheel maker? (sorry if this sounds stupid question)