I get the bus every day to the city. Varies from 20 minutes from Warringah road (Forestville) in school holidays up to about 45 minutes if the traffic is terrible and during peak hour. On average it would be comfortable 30 - 35 minutes on the bus.
On most days it takes about 30 minutes in the car during peak traffic hours to go from Forestville to the CBD or vice versa via Eastern Valley Way. Monday mornings seem to be the slowest to get to work and Thursday afternoons take the longest coming home - no reason why though! The longest it has ever taken me is 45 minutes into the CBD and 1 hour on the way home, but this was because of accidents.