If you're definitely heading to Cradle I'd concentrate on that area rather than spend the time driving. On that, beware the travel times (and local advice at times is shockingly bad... it doesn't take two days to drive from the Tarkine to Hobart but it chews up most of a day)
Presuming your starting out from Hobart, its essentially a day trip to Cradle (consider a short walk at either Lake St Clair or theres a couple of trailheads where you can walk out into the Franklin or save it for when you're at Cradle and do a quick circuit of Dove Lake).
You can easily spend two days on the network of tracks there - check out the info centre - Marion's Lookout and around the Face Track or back via Crater Lake on one day and then Lake Rodway and Twisted Lake on the second would be my recommendation.
Come back via Sheffield / Deloraine - there's a couple of short walks in that area for the return drive. Check out a booklet PWS puts out called 50 Short Walks (or something like that)
You've missed the best of the long daylight hours this time of year - daylight saving has finished and the days are getting noticeably shorter
