TBS would know better than I, but 1 hour was the figure that came to mind.
Plus you need to leave your walking time to gate and the 20 minute boarding window plus contingency.
So for a 17:00 departure I wouldn't expect a full service dining meal in a lounge after 15:00 and that might feel rushed, so I'd probably aim to be in the lounge for 14:30.
Does that gel with your own experience?