This came up in Tampa Bay's request to move to California (which was approved and will happen this offseason).
Basically we required TB to pay for their new stadium at the beginning of the current season.
What was discussed was setting up an official policy for this. Here are two opposite thoughts on this...
In the game you can move or build a stadium whenever you want.
In real life, this would take a few years of planning and construction.
So what should our policy be? My thoughts are:
1) you need to be building a stadium or moving for a good reason, not because you've always wanted to have a team in your hometown.
2) At the very least you should have to request this, and get it approved a full season before you move, as well as pay for it ahead of time also. This is a game of planning and strategy, so this should be included.
3)Two years might be better/more realistic. I'd like to make this an option, but don't want to make it to long so that people don't bother considering it.