It is a nice ride depending on your route. Easiest is to ride out on the Continental trail at Nant, head up to Angler's Retreat at the furthest point north (by Nant-y-moch reservoir) and then head out to Mach 3 via the 'Hollow trail' as they did in the big-ride-for-little-lives. Once on Mach 3 go down the Chute and head onto Mach 2 (all signed).
As for times, I've only timed it from Angler's Retreat, 30 minutes to the bottom of the chute and 1 hour and 5 minutes to the golf course at Mach :). So add on 45 minutes to an hour to get from Nant-yr-arian car park to Angler's Retreat I guess.
It is definitely worth it as a route, the sections of Mach 3 before the chute have some stunning scenery where the valley slopes off steeply to your right :). Another good alternative would be to ride the outward part of Joe's Back Garden from Nant car park and then join Mach 3 and then on to Mach 2. Takes you down the side of Plynlimon 🙂