How many days do I need for a proper diving trip to Marsa Alam?

For fun diving, 4-5 days gives you a comfortable pace (one or two dives a day with rest time) without feeling rushed. If you're doing a PADI Open Water course, budget at least 3-4 full days since it combines classroom sessions, confined water and 4 open water dives. Liveaboards are usually 5-7 nights.