The general view in terms of blog posts - which I assume applies to web comics, too - is that the best times to post are between 1030 and 1400. Lunch times are good, because that's when people have time to browse. The most important thing I've been advised is that if you start publishing a web comic you need to stick to the regular schedule you decide on, whether it's daily, weekly, fortnightly or monthly. I'm posting new episodes of "Crucible" on Thursday mornings UK time, for example, and I do the dame for another comic, "Witchcraft Street", which has its own web site and I'm curating for friends.