When is a used vehicle purchase a certified lemon?

If you purchase a used car or truck in this case, if you dont have any problems at all with it at what time frame would you consider that a good purchase?

6mths later with no problems? 1 year or what? thanks

A year would be nice.

its used and usually sold as is. i hope it lasts a while but if it blows a headgasket or a trany breaks soon after suck to be me. i should have had someone look at it better

"Problem" is a relative term. If something minor goes like a starter or alternator or something no big deal. However, if the trans or head gasket blows within a couple months, I'd qualify that as a shitty purchase.