similar to physical exercise, anything that actually gets you doing and continuing the exercises is advised.
Gains are based on how long you've been doing the exercises for and what the frequency was. if you're doing 15 min sessions one every other day for a year, (no time off) you could easily see 1~2 inches gained from consistency, from never doing the exercises ever before. if you have done them before, but not "seriously" then knock that down by a half inch. if you've already been carrying through with it then knock off another half inch. generally a full inch from year one, and a 1/4 to a half or full each year after depending on what year(total) it is.
that's the main point of contention though, is consistency.
There are genetic limits for people, and that's entirely based on that person.
