I guess it depends on what you want to do. If you're specifically going somewhere to do something then probably 2 or 3 hours maximum. However, some people (like me) enjoy road trips and if this is the case, anywhere, as long as you're back home for work on Monday morning. I like watching the scenery go by and try to enjoy the journey as much as the destination.
We will drive 3-4 hours, but recently got a steal on Tiger flights to Coffs, so flew for 45 minutes, and think that is now an optimal weekend away - great empty beaches, less time than driving to the central coast. Weirdly, we tend to stop at the Blue Mountains if going inland. Coastally, we'll go further.
There are so many great places near Sydney that you don't have to travel more than 3 hours for a weekend away, with many places within an hour of Sydney.
1 hour gets to Wollongong, 1 and half to the blue mountains and 2 hours to Newcastle. These are also 3 destinations you can go to by train and skip the hassle of driving.
3 hours will take you to the Southern Highlands or the Hunter Valley or over the Blue Mountains to the Jenolan Caves. The Caves and the Hunter Valley are great to do by train/bus, especially if you want to focus on some wine tasting in the Hunter.
You don't even need to leave Sydney for a weekend away, just head off up to Palm Beach or rent a House Boat on the Hawkesbury River. In fact many of my friends have taken the opportunity during the low season to stay in the CBD as their weekend away.
Note on long weekends, so many people go away that you can easily add an hour or more to your journey making it quite hard to get away anyway.