User blog comment:Don7/Luffy can live more than most humans(?)/@comment-1889545-20121111102616

Luffy's cells will still contain a Hayflick limit (the number of times that any given cell can divide so long as it isn't a cancer or stem cell) and undergo apoptosis (programmed cell death). While his rubber body will protect cells from impact related trauma, sharp objects can still damage him and he can take damage from other sources as well. As such, he will probably have an average lifespan regardless of his powers.

As such, your guess of 300 might be accurate if Dr Kureha calling herself young at 141 is true.