You don't get booted for not posting (I think). At most, if you just leave the rest of us a message saying you're gonna be gone, that will be enough.
Yuppers.
At least I certainly don't have the permission to boot anyone off the alliance... I believe that's something Auron/Rug and SS decides.
And most likely they'll boot only for the extreme cases (traitor, enemy, attacking non-orcs without them starting the fight), not for a mere vacation or not posting...
In the case of prospective recruits, though, that's a different matter:
If over a month passes by without them posting a thing, they deserve to be booted off.
It's a clear evidence of not committed to the game.