I think it's rude to hog the loo with applying makeup etc for too long. I'd say about 5 minutes would be about the polite limit. They could even choose to do this at a quiet time of flight instead of when there is likely to be a queue.

If someone is "needing" to be on the loo for a length of time due to their physical needs, that's completely fair enough, though, and I think they should take as long as they need to take.