Largely depends on time of day and which way you travel. Peak hour Mon-Fri for example is in line with what Katie1989 states in her response, however outside of peak hour, it can easily be achieved in under 20 minutes via car with a good run of lights. That's timed from Mawson Lakes Blvd, through to North Terrace via Main North Rd. When the Northern Connector works are completed, from what I can tell, it appears that you may be able to depart Mawson Lakes, making your way onto the motorway shortly after, and have your entire trip to Adelaide free of traffic lights, being able to travel at 90km/hr for most of the trip. Too early to say, but that may mean travel times from Mawson Lakes to the Adelaide exit on the motorway, in not much more than 10 minutes. Very much looking forward to it. Expected completion is late next year (late 2019).
To travel from Mawson Lakes to the city takes an average of 26 miniutes but it can take 35 minutes. However, I would leave with 45 minutes to spare to ensure that you get to where you need to go on time.