Oxygen Needed for 15 Gallons

For my next batch of RIS I plan on using pure O2.  I am looking for some guidance on how long of a shot of O2 do I need.  Also, since this will be a big beer, I am thinking of giving it a shot of O2 8 to 12 hours after pitching.

My thinking is 2-4 minutes slow trickle per five gallons.

For each 5 gallons, I usually go 1 liter/min flow rate times 1 minute prior to pitching yeast.  For a beer like an RIS, I may hit it at the same flow rate for 20 seconds or so withing 24 hours of my pitch.

15 gallons of RIS?  Mmmm.  If it’s all-grain that’s one big mash.

I would agree with ~2min per 5 gallons and repeat 12-24 hours later.