I have done the Rea Lakes Loop 7 times, 6 of them were in one day
A bit of information I can give you is DO NOT GO CLOCKWISE!!!
I have done this once and it is horrible compared to c/c.
You have to do a painfully slow push up hill for over over 30 miles with a tiring hard on the knees downhill at the end.
Going clockwise on a 2 day trip is better because you wake up and have a little push to the top and you can slow down and enjoy the down hill.
The only problem going counter clockwise are the gajillon steps you go down after the upper campground.
Going counter clockwise gets you up to the top of Glen feeling like crap. You think you are going to feel better once you drop down to the lakes but you don't until you get to the draw-bridge/ JMT split. The rest is easy. You will also get to the top of Glen by 1pm.
Going clockwise means that if you do not feel good and have to slow down even a little, you will be getting to the top of Glen with not much light left.
Okay, so going clockwise is an easier go getting up to Glen. If you do go clockwise, I would leave early, like 3am early to give yourself the margin to slow down and still get below the tree line by night fall.
I have gone counter clockwise in a dry year June day and made an easy trip of it not even having to use a light the entire trip. This was due to the easy miles of downhill that I averaged about 3.3 mph going down and about 14:45 hours in total time. Usually it takes me about 13-16 hours but I usually slowly jog at least half the way down.
You can expect a clockwise trip to take you at least 17 hours and I would say more like 19. It takes a looonnnng time to get up to Glen going clockwise.