VPIP & PFR accumulate every hand, and therefore converge more quickly than almost all other stats. A sample as small as 20 hands will pretty much tell you what you need. With 50 hands on someone, the VPIP/PFR is reliable. After all, the difference between a 33/15 and 34/16 is pretty slim anyway.

The only time this is not solid is when someone get an extreme card distribution in their first 20 - 50 hands. Like getting a ton of great hands in a row... or the opposite, running card dead for the first few orbits. This quickly resolves itself and your Spidey senses will tell you that something is different about that villain until the stats catch up with them.