I would begin to trust pre-flop stats (VPIP & PFR) after 50 hands, but only if they're one on extreme (loose/tight, aggro/passive) or the other. Post-flop stats I wouldn't even look at until you have at least 100 hands, and I wouldn't make any big bet decisions based on them until you have at least 200 hands.

As for non-stat/HUD based reads, these are useful immediately.