External pre-flight - normally around 5-10 minutes. This is a pretty thorough and structured check of the entire airframe.
Internal pre-flight (pre-engine start) - anything from 2-3 mins when solo to 10 or so mins when new pax are on board.
Take as much time as you need to be 100% comfortable. Agree with tacpot, I've seen some very 'relaxed' pre-flights. The overall standard could be improved IMHO.
l_b