As said V, the cranking time will have nothing to do with clamping the lines - there might be a little bit of air gets in the pipes if you don't clamp them, but if you don't fill the filter you've got half a litre of air (so to speak) in the middle of the pipes!!
I didn't clamp or fill the filter and was fine. Doesn't feel right to crank it for so long though, can't be good for the starter motor. I was going to fill it but forgot to stop for derv at the garage and couldn't suck any out of the tank.