This circuit has lots to offer, and is a fantastic three to four day walk. How long it takes depends on what you want from the walk and what interests you and how long you linger at each attraction. Your plan is very doable.
There are heaps of waterfalls in this area, all spectacular and all worth visiting. Starting from the end of Mersey Forest Rd you can head up the Moses Crk. track to Grail falls on the southern end of Chapter Lake. If you are use to off track walking, then up the valley to the west of the Moses Crk trk there are two waterfalls (Horeb and Meribah falls) and a number of cascades to visit - as detailed by tastrekker's post on his old blog - unfortunately I can find the forum topic or a report on his new blog.
Corvus and the Strollers have walked the
Mersey Circuit. This is the trip up the Moses Creek Track to Junction Lake and Clarke falls, the Never-Never past Feather falls and McCoy falls, to Hartnet Falls, along the overland track past Ferguson, D’Alton, Boulder and Cathedral falls and then down into Lees via Kiaora falls and Ladder falls, back to the Mersey Forest Rd past Oxley and Lewis falls. I've also done this trip along the
wild mersey and placed some pictures in the
gallery here. With 15 major falls and a few smaller one, I reckon this is a must do trip.
One tip, if you do go looking for Ladder falls, the Ladder Creek is not hard to find where it crosses Lee's paddock, and the bottom cascades and smaller falls are on the edge of the paddock. There are about six small falls in this cascade. To see the real Ladder falls you need to climb above the cascade and walk upstream about 100-150 meters. According to my gps Ladder falls are 89m tall, but I doubt that is accurate. Still, if there is plenty of water in the lower small falls, then the climb is worth while.
Here is a Google Earth view of it all.