If you've ridden a lot in the past you'll probably be able to tell how a bike is going to ride in the long run with just a couple miles of test ride. I was road bike shopping about a year and a half ago and probably rode most of the bikes for ~15-20 minutes, usually pretty hard around the neighborhoods the shops were in. I probably could have ridden them longer, but that was generally enough. Budget Pro did let me take the bike through the neighborhood-- they didn't say anything at all about how long, and I went up and down some of the local hills.

The most important thing for me in test riding a bike is getting it set up right before I roll out of the shop. I bring my measurements and try to get them matched on the stand before I roll.