What kind of bike are you riding?
In a group, riding roadies and favourable winds and good fitness 3hrs is feasible.
If you have average fitness and are tagging onto some groups but get dropped on hills, maybe 4ish hours - includes lunch stop.
If you have poor fitness and walking the hills... allow 6 hours.
On an MTB with MTB tyres, the average punter with average fitness could take 5+ish to 6ish hours.
There are some great threads on here regarding typical training kilometres leading up to the ATB. At my most fit, I rode 100km on my own (from Altona Meadows to Black rock and back via Williamstown) in 3h 20m... I'm a long way off that fitness today though
It's much more fun to do these rides with a buddy though. Even side by side you get some aero benefits.