Just to toss some math out there since I'm bored, since your tanks and probably mnk will need it way before drg.
Drg is going to consume about 350 unrecovered tp a minute in a training dummy situation.
With no invig they will run out at 3 minutes.
Maintenance invig (start at 2 mins every 2 mins) they will last till 5:40 mins > +4 ticks anywhere in there adds 2 mins > repeat (+6 = 2 mins > +7 = 2 mins > +7 = 2 mins)
Reactionary invig (start at 500 tp every 2 mins) they will last till 6:50 mins > +4=2mins > repeat (+6=2 > +7=2 > +7=2)