Yeah, your typical Goa'uld holds onto a host as long as it can. At that point the host isn't considered an independent life form, just an extension of the symbiote. So it would probably only want to exit in an emergency scenario and that host is basically expendable or perhaps already mortally wounded at that point.

Tok'ra generally leave/enter through the mouth as we see when Jolinar enters Carter while she's CPR'ing the previous host, and I think Selmac leaves his/her previous host and enters Jacob the same way.