dalegphoto wrote:I was wondering if it's possible to do the trip there & back within a day leaving from the Dove Lake carpark & if you've done it how long it took
Very achievable, depending obviously on the ability to walk at a decent pace, and get away early enough. In December you'll have no lack of daylight hours, just remember that access to Dove Lake is bus only now and they don't start until 8am or so, which is kinda late. Get in at 6am (or earlier), drive and park at Ronny Creek... it's only an extra 2km along the road to Dove Lake. Or alternatively take the start of the actual track from Ronny Creek via Crater Lake, bit longer distance to cover but the climb up to Marions is less intense.
As a guide... YMMW of course... I've been able to get out to the Barn Bluff track turn off in around an hour or so, but admittedly that involved a bit of running. Managed to get to Waterfall Bay in 2hrs15min with a fairly heavy pack once, but that was in a minor blizzard, on my own, so I wasn't mucking around

Maybe 2-3 hrs would be a realistic allowance to the turn off for most capable walkers, then maybe another 1.5-2hrs to summit and return.
If you're going to go on further south to get that particular angle, we managed Dove Lake to Lake Will in a shade under 4 hrs at a more sensible pace (doing Mt Inglis in a day - 16 hrs) with daypacks. Most of the track from Cradle Cirque down past Lake Holmes is boardwalk/stairs, so expect to manage a comfortably good pace.
Good luck with it. Scoparia in December is one of life's simple pleasures.