More time would be better.  I suggested 6 months because there's a new YC cycle that often, but a year would be better.
Though in fact 6 months of working together is probably enough to bring most problems to the surface.  Just make sure you work together on stuff that's demanding enough to put some strain on your relationship.<p>It's more important to have a good cofounder than to have one with particular skills.  So if I were you I'd simply try to find someone you work well with, whether he/she's a business person or not.  Two hackers can figure out how to do sales eventually, as long as they don't give up.