In reply to FesteringSore:
If you are fit enough it can be done, however probably not by the nicest route. There are probably more enjoyable day walks to be had in the region.
If you are happy to sleep out a night, the Trevelez-Siete Lagunas route is very nice. Bivi cave and great wild camping spot a t Siete Lagunas.
Edit: Actually, thinking about it, if you are happy to walk a bit in the dark and are experienced enough it can well be done in day by the Siete Lagunas route as well. It's a very long day though and the altitude does slow one down.
Post edited at 16:37