So ive heard ~1500mi is about when to replace shifter cables, especially shimano?
thoughts?

That's nuts. Every year or two should be fine depending on how often you ride in bad weather and how often you ride. Just check for wear and evaluate how smooth the shifting is, then change them as necessary.

My schedule :
5000km for cables
10000km for housings

znomit has it about right. If you ride in the rain a lot it might be a little more frequesnt

I do housing about once a year. Cables get replaced when they are worn out - anywhere from 6 months to a year.

But, to put that in perspective, I ride between 7,000 and 10,000 miles a year split over two bikes (one is a dedicated race bike, though I sometimes ride it during the week when I am being lazy.)

I change cables and housings when shifting degrades, except for my Ultegra 10s. On the Ultegra, the right cable breaks at about 3300 miles, so now I change it at ~3000 miles. YMMV

10,000kms on my current SRAM cabling. Works as good as new, but then I don't ride this bike in bad weather.

I probably have about 11,000mi +/- on my Jagwire cables/housing, running 2007 SRAM Force. I have no idea how long they'll last. I guess I'll replace them when they quit shifting smoothly.

Once a year, both sets.

On Shimano (external routed shift cable), I inspect the right side cable (rear) regularly for fraying. I've had two of these break on me. The breakage is always preceded by a degradation in shifting precision.

As often as I overadjust and fray the ends. I need to stop doing that.

When the shifting starts to degrade. I don't know in terms of miles, but it's a hell of a lot more than 1,500 of them!

How about internal cables same time?

I had that happen a few months ago. I had to rip that sucker outta there, but that was before I knew saw what the problem was. I'll probably do it a little more delicately next time.

I'm supposed to replace cables if they're not broken or damaged ?

External cable Shimano 10sp breaks cables quickly. There's a sharp bend inside the shifter, so the cable breaks there.
How many miles you get depends on how often you shift. I shift a lot and mine go about 3000 miles for the right shifter.

You can pull the brake lever and shine a light in the cable acess hole after shifting to the large cog. If it's fraying you can see it.

Me too.

Recently replaces my rear shifter cable, after 12,000 miles. This was a standard Shimano stainless cable that came with my shifters. I inspect both cables whenever I do a drivetrain cleaning (~750 miles). In my experience they start fraying either at the rear deralleur or at the shifter, rarely in the middle (maybe under the BB, maybe). Catastrophic cable failure is rare in my experience, it's usually gradual fraying.

Cables and housings gets replaced once a year for the bike that goes in slop, less frequently for the bikes the ride mostly in good weather. There's no good reason not to replace them regularly. They're cheap, and it really sucks when they break.

The type of failure I see most is the head pulling off. This won't be visible unless you really look for it.

Since I became the dedicated bike mechanic within my group of friends, I purchased file-boxes (bulk) of cables and housing. I've only been riding for a couple years now, but I'm making it a habit to change my cables and housing at the end of the wet season here (SF Bay Area / around April).

I was just working on a friends bike a couple weeks ago. Shifting was horrible. Cables were rusted. Housing was in poor shape. Brake pads were worn down badly. Rear wheel was way out of true. Derailleurs weren't adjusted properly. Chain was caked with grease.

I asked how often the bike got a tune-up. He said that he bought the bike for his son about 20 years ago, and the only thing that had ever been replaced were tires and tubes. When I got done with it, he was blown away by the difference a tune-up can make.