Board Thread:Naruto Discussions/@comment-25977287-20150704114842/@comment-734582-20150704185139

JOA20 wrote: Not once have I read that one can choose to gather different amounts of natural energy to enter Sage Mode. One just balances natural energy with one's physical and spiritual energy, and stop. Minato doesn't have as much chakra as other Sage Mode users, so it sounds right that his Sage Mode doesn't last as long as theirs. It's stated in the Sage training that one needs to absorb the right amount of nature chakra, as too little does not give Sage Chakra and too much kills you. So you can chose to gather as much as you want. I doubt you need to gather an exact percentage (so there's a bandwidth to work with) so you can either choose to gather the max bandwith or the minimum of the bandwidth (and anything inbetween).

It's also likely that Minato indeed has less chakra and so doesn't need to gather as much.