I don't mean finding some optimal strategy, I mean how long should it take a normal person using standard techniques such as doing edges first?

Assume NxN to start with, and roughly square pieces (so no clues about orientation). Can discuss extreme cases (at one extreme pieces numbered, at the other extreme all pieces the same colour). Also discuss easy bits, such as writing (as long as there's not too much of it), and difficult bits, etc. etc.

Bring in randomness, so looking at expected time as a function of N. Can have different parameters representing ease/difficulty of different types/areas of picture. So each jigsaw might have a difficulty rating besides just N.