Getting out of the water into a boat is a different matter - but still a long session overall. And to be in an open-cell wetsuit. Quite a committment with no toilets nearby! I think the most I have done - without getting out of the water - is about 4 hours and that seems enough. Although I suppose folk waiting on shore for me is usually the main time constraint and probably after than tiredness (if dealing with currents), dehydration (I carry water now though), seasickness (in choppy conditions/swell) or poor conditions (no viz., no sea life - although I've learnt the latter should be ignored because sometimes good fish show up after a few hours of nothing).