My sessions are about 20 minutes. Settings vary between 20-30 (she said 30 was the max for the machine used)
My suggestion would be to get as much as possible off with laser, finish off the rest with electrolysis, then HRT. You wouldn't start mowing a lawn with the grass clippers, you'd use the mower first, then the clippers for what remains.
Laser (relatively painless, fast, and targets a broad area (but maybe misses greys))
Electrolysis (more painful, slower, targets individual hairs (excellent for getting those missed greys)
HRT (hair changes are a beneficial side effect, but one doesn't do HRT *just* to lose hair)