I run a custom 5 oz tank from Walt in my .12 rigger. At the end of a heat I have about 1 to 1.5 oz's left . I know guy's that run a 3 oz tank but they launch at about the 40 to 35 sec. mark and just barely have a drop left after the heat.
In 2007 I ran about 3.5oz and if I blew the start I would be lucky to finish with starting at 1:30. In 2008 I didn't want that to happen so I ran just under 6oz and I could run what seamed like forever, that was much nicer never had to worry about the clock.