The best way to keep an eye out for people is to add them to your friends list in the social panel. It should tell you then when people come online, and if they are green on the list, they are. (Sorry if you already know this, it just took me a while to figure out.
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I know I can't really plan when I'll be online, only becase I have to play on the family computer. I only really have to fight with my brother for it, but sometimes my dad as well. So I'm used to finding myself disappointed when it comes to planning. Sorry!
Typically I'll be on in the early evening on weekdays, and the early half of the day on weekends (EST), if I dare give a usual.