how many draws do you get on one battery on lets say 3,5ish ?
Depends on the battery, the age of the battery (amount of use more importantly).
* the biggest thing it depends on is the type and length of the draw*
I remember I tried doing this a while back, I think I got through 6 chambers off one charge, mostly multiple hits but a few single hits that killed the chamber. I'd have to find the post I can barely remember making it or any of the after math. It was over 2 minutes of run time though, which when you're choofing back thick clouds is fairly ample for quite some time.
Overall the battery life is bearable, but with my GHB1's its a big downside, any battery tech is always going to be a downside IMO. However the hopper could be more impressive, and may be eventually or already with the GHB2s
I also basically always attempt to make the biggest clouds, and almost never use it directly.
Direct use is a lot kinder on the batteries, and if you supplement your day with it it works pretty well. If you put it through its paces, it won't hold back and after some solid rips the battery will give up.
I've also had an entire session with a 'flat' battery, where I kept forgetting to replace it but it kept giving me hits about 3/4 size of the biggest possible, so I simply kept forgetting to charge it.