How long does the impression tray stay in when fitting for a crown? What if I gag easily>

2-4min. It depends on the material that is uses for the impression but many new fast setting materials will still take a couple minutes to set properly and for a crown accuracy it is critical to be completely set up before removal which could cause distortion and a poorly fitting crown.
Depends. Material setting times can var between 45 seconds and 5 minutes. To help some one who is a gagger, a short set time with a small tray should be used.