It's about 20mins @ 7:30am and up to 30 minutes at peak hour. I work in the southern part of the city so I take the tram which is 25mins. If you work in the northern end of the city eg Rundle mall you would catch the train.
Approximately 20 minutes outside peak hours. But even if you are commuting within peak hours and will have to factor in some extra time, the traffic flows quite well on all main roads, and your total time on the road will still be relatively short.