Depending on the gel strength you might be wearing them anywhere from 30 mins to overnight.

Gel strength will depend on sensitivity potential, and whether you have 2-3 weeks, or want it done ASAP. Slower approach tends to be longer lasting and less sensitive.

You might also be later inclined to do composite bonding or veneers, but whitening is a nice conservative step to take first. In my market, this costs about $250, though I hear of some systems (presumably with a considerable amount of monitoring and follow-up?) that carry a fee near $1K.

You will also see whitening offered with incentives, as it is one of the few things other than a power toothbrush that has nearly universal appeal and doesn't take a lot of doctor chair time. That might work just fine, but there's a chance that your tray fitting and gel options may not be quite the same as if they were paid for, rather than comped.