How often for a new chain?

Start of the season, every year along with a tune up and drive train cleaning

Depends on lots of factors...you can learn to measure it with a ruler or buy a tool to measure it so you can learn what is normal and how long your chains last.

I replace mine usually 2x a year (ride about 4500 miles less this year) as I have multiple bikes, wheels and cassettes and keeping newer chains on my bike keeps everything wearing together better as I swap things around between bikes and wheels.

With the current crop of 10-speed gearing, you should be replacing it about every 1,500 miles or so (again, depends a lot on how you take care of it and the sort of conditions you ride through).  Could you go further?  Sure.  But then you end up wearing your cassette/chainrings faster.

x2  This is close to what I follow.  I'd rather drop dough on a couple chains, than a new 1090 cassette. 

Edited by BamaDC 2009-06-24 12:58 PM

When it needs to be. How do you know it needs to be changed? I use a Park Tool chain checker (a ruler will work as well) to indicate when it is worn. When it is I change it. I start checking every couple of weeks after I have 1000 miles on the chain It usually lasts around 2500 miles. I do clean and lube my chain every couple of weeks or after I have ridden in the rain. About every second time when I replace the chain I also replace the cassette. This avoids shifting issues using a new chain with a worn cassette.

It is not just about the chain stretch in a linear fashion, it is about the side to side "slop" that you get.  Makes shifting crappy.

Thanks everybody.  Good info.  I could've just PM'd Rick but I wanted to see if there were diverging opinions and apparently there are.  He did check with a Park Tool.  It was loose, but not too bad.  But the gears have definitely been grinding this year. 

What's the best chain-lube?  I'll get something better than the Schwinn WD40 knock-off stuff I've been using.

Finish Line makes some good stuff in general.  I personally use a different product called "White Lightning" as it is not so gooey and the dirt and grime don't stick to it so much.

WD40 type products = very bad .......

Thanks Rick.  I'll get some of that.  And Zinn while I'm at it.

Was just lectured by a LBS last week that 1,000 miles is the mark.