Qie there's absolutely no reason to even mention how long you can leave your group without the right amount of tier, especially giving the impression there is no rush. This could quite possibly lead to future land losses for owners and be directly as a result of what you say here.
Shorahmin, the post by Lina has the right of it, group bonus may help. However if you choose to use Qie's more complicated method, please do not rely on some unknown timeframe for leaving the group without enough land credits, get it back to normal as soon as possible.
There is another alternative. People let you 'rent' tier from them. I'm sure if you do a search of "rent tier" or "hire tier" you will find such a service.
~B