As an intermittent player (mandolin) I find myself frequently in "virgin tips" land, and the best method I've found is just to play for not more than about ten minutes, then do something else for twenty or thirty minutes before repeating.

About five or six ten-minute "sessions" is generally sufficient to be ready to play all day on the next day. I try diligently to practice at least twice a year, and it works every time for me.

For some it might take a few more cycles, and after the first half-dozen cycles you probably could try for 15 or 20 minutes of play at a time. It may take more times around for "more agressive" players.

Trying to rush it by playing for a half hour the first time I pick up the weapon makes the tips so tender that it takes three or four days of healing before I can start over and do it right. Once it hurts, there's little you can do to recover healthy meat to hang the callouses onto, but it doesn't require an extended rest between brief playing breaks as long as you don't cause pain at the start.

Of course, if you've already hurt yourself you'll want some sort of protection. You can't create a callouse where there's already a scab.

John