I think 10 minutes is the bare minimum, as in running full throttle for 85% of that time. However, I havn't used the z-drives so I couldn't help you out there. Probobly your best bet for runtime now that I think about it would be 4 batteries of the highest possible voltage. Therefore, the boat would be so fast you wouldn't run it at full throttle very much.

But one thing for sure is that running 75% throttle for your whole run, you won't see much more then 10 minutes.