Depends on what time. In the morning a lot of people are traveling towards the city for work, so the lanes leading into the city are usually pretty busy from about 7:30 - 9am. From the Plympton area you are looking at about 20 minutes to get to the city during peak hour, 10 minutes any other time of day. The same goes for the lanes leading away from the city in the evening, usually 4:30 - 6pm is peak hour.
The road is busy, however its not 'Sydney' or 'Melbourne' busy - traffic still flows pretty readily.