Hi Graeme,
My longest moon AVI I have recorded was 10 minutes at 60 fps.I got bored and the AVI was HUGE and it took forever for registax to process it.You can get such short exposures on the moon that you freeze the seeing very well and I didnt see much difference between avis that were 2 minutes or ones that were 10 minutes in length.How good Seeing was was far more important than number of frames.