as title says, i am never reall sure just how tight i should go? dont have a torque wrench but after about two hours riding the bolt that holds it all together works loose, have tried lock tight but still it happens. have a CK bottom bracket set so dont really want to lean on the allen key to tighten as hard as i can.

any advice happily recieved, thanks

I had this problem two weeks ago then at Selkirk event it fell off and wouldn't fix back on.
After buying a new XT chainset I then discovered I'd got too many spacers in bottom bracket. BB is 73mm, not 68mm as I read on detail.
Not had problem before on other bikes with Race Face chainset so this was my solution to same thing.

RF cranks tighten to a stop on the axle, if you don't really lean on the allen key then you won't fully tighten it, and you'll then run the risk of crank damage.If your BB shellis faced to the correct width then you won't be able to damage your BB by fully tightening the cranks, the stop on the axle prevents this.

I had the same problem.

Make sure you have the two plastic washers that fit on the drive side (one red, one black), they sit between the crank arm and the BB. I'd lost one so no matter how tight I tightened up the crank it still wobbled lose after a few hours of riding.

I got sent a new black one from a kindly soul off here FOC…!

Tighten up the bolt then just tighten up the out cap nice and tight as they are counter rotating they shouldn't be able to come loose.
Ensure you have the all the right washers installed.

http://www.youtube.com/watch?v=XT3nH0mxPCY&feature=related

so dont really want to lean on the allen key to tighten as hard as i can.

This is probably whats actually causing your problem. You need to tighten the cranks until they hit a dead stop, assuming you have it set up correctly then this will not damage your BB at all.