Straight from here, minimum is 5ml per roll.

I've only ever used rodinal at 1+100 stand (or semistand, max 3 agitations in an hour.), so far on APX100 and PanF50, but mostly on Rollei Retro 100 (I scored a 100' roll cheap). So that's only 3+300ml per 135, and 5+500 per 120 roll, never had any problems with it becoming 'globally exhausted', but then that's the point of 1-hour stand, in that it becomes 'locally exhausted' and doesn't blow your highlights.
Of course, even using proper agitate-every-minute, it'll depend on how much of each frame is light or dark as to how much it gets used up.

5ml/roll sounds a bit like a 'best before' date on food: it's there to protect the manufacturer from whingeing customers, not to protect the consumer, so it's probably very conservative.

How much film have you got? As always, start by following the manufacturer's recommendations and then you can play around. You can get away with using less, but it's probably the cheapest thing out there so there's no point in saving money with less dilutions, only do it to change the effect on the photos.