[ottsville]

You can start looking for glaringly obvious bad play at any number of hands, but if you are going to start doing that through statistics from PT, you will need A LOT more hands to develop an accurate picture. Some parts of the data (VPIP,PFR for example) will start to attain an accurate statistical average more quickly than other parts(winrate, 3bet stats, etc).

Try doing session reviews if they are offered in the appropriate forum for your game.