When I feel like it. Is there a better time?
No, no there isn't.
I have rewatched Farscape and Firefly at least a dozen times. BSG, Babylon 5, Buffy, Angel, Stargate SG-1 and Atlantis...I've watched all of those at least 3 or 4 times. I just do it whenever I want.
Sometimes I will rewatch part of a series and take a break for a while before continuing, but I love rewatching my favorite shows. You get so much more out of them when you see them again because you catch a lot of things you missed the first time around.