This is another thing where you have to ask your surgeon, I think. Brassard's schedule is 4x a day for 20 minutes for the first month, then 3X a day for 30 minutes for two months, then 2X a day for 30 minutes for three months, then 20 minutes once a day for six months, then taper down to 15 minutes once a week for life.
In other words, the absolute worst case with Brassard's recommendation is either 90 minutes total per day (months 2 and 3) or 4X per day (first month). Of course, that's the total time the dilator is supposed to be inside - I guess I'm lucky, because it takes me no time to reach full depth and about ~5 minutes to set up/clean up, so I don't consider those to add much time at all to the sessions.
Ditto for keeping a dilator inside; I admit, I think if that were a good idea, at least *one* surgeon would recommend it. Personally, I'd be worried about infection (the dilators are supposed to be sterilized after each use, after all) and that early on the healing tissue might heal *around* the dilator and make it difficult/painful/damaging to remove.