"How long is the trip?"

I don't think you need to say "for" in For how many days is the trip? How many days is the trips sounds better. Now you can ask the question in a different way if you planned to go on the trip with the person. If you are going on the trip, ask: How many days will we be gone? Or When do you think we'll get back (return)? If you're not going on the trip, ask: When will you be back (return)? I don't like the suggestion: how long is the trip Because a person might respond, "It's a three hour drive." That answer might be helpful but it may not be what you are asking about

Grammatically correct yes, but it sounds strange for me (Michigan, US) How many days is this trip?, or I would prefer, How long is this trip? We will be gone (...) days. The trip will last (...) days.

"For how many days is the trip?" Perfectly correct. Answer: The trip lasts [number] days.