Yup, that's about right. I get about 6 minutes on my dual 11.1V packs with mild 3D. 7 to 8 in normal flight.

In my opinion, you should do as Shag said, and get 2 11.1 packs instead of a single 22.2 pack. It's a bit more work having to charge two packs for a single flight (I've got two chargers so it's no big deal for me, but not everyone does), but It's just more convenient. You can use the packs in other smaller models other than just the T-rex 600, and, as shag stated, if something happens with one of the cells, you're only out half the cost rathern than the full cost.

Here's a shot of my helicopter, in the bottom right corner you can see my 2x Electric Power 4400 mAh 12C packs connected with a series connector I made.

Multi-packs is the way to go!