Do you think it would be rude if I ask two people when they are going to leave their booth at a cafe.
Why would you want to know when they're going to leave their booth?
I need a place to sit, and I've been watching them just sitting there for a while.
While it's understandable that you want to sit down at a cafe when it is busy, it can be rude to ask others when they are leaving since it puts pressure on them to leave. Only cafe workers should request patrons to clear a booth. You should try to find alternate seating, such in the outdoor area, if you can't find a spot to sit at the cafe.
I just want to know what the seating situation is, I've been looking for a seat for such a long time.
You might try asking one of the workers about the wait time. They usually don't mind giving you a guess about it. They may also be able to encourage people who are finished eating to wrap it up.