Well, what time would suit you?
Set up runs at a time that suits you and see how many can join you then to give you a better idea of what works. One time won’t fit all anyway. And then, you can change time after few goes if it doesn’t seem to work.
Keep in mind, only a few guildies actually look in forum anyway. So, why not try posting next day runs on Guild Wall in game to inform people when you will be around and hoping to do some experts? Might work.
I will sign up on forum if I think I will be around even if that end up to be a last minute thing.