What is the minimum time needed for the visit if you want to see everything?

2 to 3 hours but if you want to enjoy beautiful Sun Rise you have to stay overnight.

2 hours should be enough. I will strongly suggest overnight stay at the cottages. That way you can make it for both Evening and Morning Aarati. They have a good Cheap Dinner option available as well. If weather allows, there is a beautiful Sunrise view site by the temple.

Temple - about 30 mins. But if you want to Actually look around and go to the Ganga park -around 2 hours