I would say about 30 minutes, although a lot will depend on whereabouts you get to park at Universal's multi-storey parking lot (which is free after 6pm, btw). It is a 5-10 minute walk from the parking lot to CityWalk, or you could pay for valet parking which would get you closer.
Margaritaville doesn't take reservations and there is a cover charge to enter ($4-$6?) during the evenings.