Never.

I was going to be snarky and answer every 2.17 meters.

Almost never.

Joining the popular answer...Never.

I have only replaced tires in pairs to date. I guess if I had to replace just one and the other was in really good condition I would follow advice and put the good one up front, however most likely I would put on a new pair and keep the old "good" one as a spare, so if I had another tire need to be replaced before its pair I would just swap one out till I was ready to get a new pair.

At the end of the day, I don't find the cost of bike tires to be much compared to the cost of my alternate commuting options.

Wow I was just thinking about this the other day, since this is the first time I've actually seen appreciable wear on a bike tire. My current GP 4 Season 25c tires that I put on last August have about 1500 miles on them, and the rear tire is definitely showing more wear. On the front I can still just make out the seam running down the center of the tire. On the rear it's worn well past that. I was thinking about rotating them, but after seeing this post I think I will leave them be until the rear tire is worn out totally.

So another question, how do I know when the tire is done? There are no wear bar indicators like on car tire tread, that I can tell anyway.

I run the rear tire until I see the pattern of the threads showing through.

Continental GP4000's have a pair of dimples molded into the tread at one spot.
The idea is when the dimples are gone, it is time for replacement.

Interesting how many nevers there are. I personally wear my tires down low enough (on my commuter; touring is a different animal, since I don't want to be stranded somewhere) that I have a decently good chance (like, 20%, maybe; it has happened once out of the 5-6 tires that I have worn out since I started commuting) of getting a flat. Since I prefer that to happen on my rear wheel, I rotate so that my freshest tire is on the front. I am lazy about bike maintenance (singlespeed commuter, friction shifters on my touring bike so that I don't have to be as religious about derailleur adjustment), but this is an area that I do not mess around with. That said, perhaps most are quicker to toss a tire as it nears its end of life, so they can get away with it.

+1 (especially the part in bold)

Never. I may move the old front to the rear when it comes time to replace the rear, but if the current wear is any indicator, that won't be for at least another year.

This seems right to me for rotating tires...

I think buying in pairs is a bit silly. When my rear tire is down to the threads, the front tire is barely worn. If I were going to throw that tire away for that much wear, I should have thrown the rear away 3000 miles ago. I have no problem moving the front to the back and putting the new tire on the front.

I found a tire I liked and bought 6 of them when they were on sale, so I'm good for probably 4 or 5 years. I keep them in the basement in the dry and dark. I've kept tires down there for 10 years and they're still fine - racing teams intentionally age their tires several years, as long as they're kept dry and out of the light it's not a problem.

Another vote for never rotate. I replace tires as needed.

When the rear tire has cords showing.

You're a lot more likely to crash from a flat in front than in back and therefore want the freshest tire up front which in turn implies never moving a tire from back to front and always installing a new tire on front (then moving the front tire to the rear because it's a waste of money to throw it out with over half its life remaining).

Lol nice