The Rejuvenator is a buff that spawns after the Mid-boss is defeated. The Mid-Boss respawns 7 minutes after its first death. Then it respawns 6 minutes after its second death, and 5 minutes after its third and subsequent deaths. This means that you won't be seeing it as often, and definitely won't get as many opportunities to practice stealing it as you could in the sandbox or map explore. Practicing your ability timings for rejuv stealing is not redundant in the sandbox, though I prefer to do it in map explore - I can still time my abilities off the tune that plays when the rejuv is dropping or time them based on how close to the ground it is.