I went from a 1 hour minimum to a 1.5 hour minimum to a 2 hour minimum and then I ... well, I don't want to spoil the ending.
I found I enjoyed the longer encounters, and having a multiple-hour minimum allows me to maintain a lower number of dates per year without sacrificing income. Win-win-win, you know?
Basically, it's what works best for me mentally and financially; I may be skinny, but I ain't starvin', if you catch my drift.