Well, I have two issues with that: first is "basic demiplane" to me is talking about the original casting, not any subsequents. Secondary to that is: how long does it take for the expansions to happen? It doesn't say that either.
EDIT: Even if it takes as long as the original with every expansion, that's only a week. There are a lot more weeks than 1/year.
You know, maybe he should just do it once a week. He has other stuff to cast there anyway -- TC for seeds, plant growth, etc. I dunno... it doesn't really matter how big it is, I guess. Whenever he stops by, if it isn't currently expanding and he has an extra slot, he'll fire one off.