FWIW, a close-but-bad fit doesn't start to hurt me until about 20-30 minutes into riding. It sucked when I got half an hour out and started hurting bad enough to turn around, then had to suffer for another half hour just to get home.
There are a few online fit calculators out there; Competitive Cyclist has one. Get a friend to help you measure everything (and I mean
everything... so they should be a good friend..
), measure several times, and see what numbers they give you.