Our singer normally does all the bookings and he would definitely charge more if a venue wants us to be there longer than the 2 x 1hr sets. If on the night we decide to play more because we're enjoying it so much we just go for it until we run out of songs.
I have pushed hard for both bands to stick to just 2 x 1hr sets but its difficult when your guitarist just wants to play all night every night.
Dave