If there's a group activity that's prearranged and set at a specific time, I like people to do their best to get there on time.

If you can't make it, let people know, so that there isn't a lot of standing around and waiting and wasting other people's time. At least alternate plans can be made. RL -is- more important, but surely you value your online friends enough to at least say "sorry, can't make it on time today."

Unpredictable stuff happens. A few sporadic late arrivals is no biggy. When it's chronic, that's a problem.

10-20 min tardiness for something that'll take a few hours is fine. Past 30 mins is pushing it for me.

If we're all going to be casual about it, then say so. "Let's try to meet at 8pm, and if we've got enough mass, then we'll do this and that." That's fine. Maybe we'll delay it until 10pm, whatever. If I need to leave early, I'll go.

If it's "8pm - such and such a raid / task force, etc," then by 8.15, everyone who wants to participate had better be there, and the thing should be off by 8.30ish.