How long do I have after my decree application is approved to accept it?

Can I leave it in approved by not accepted state for years?

I belive you have to accept the approved decree within a few months of approval. You also have 1 yr from when you got approved to be a bonafide resident or the approval will be null. This probably varies

the approval letter states how long you have to unconditional accept it, not accepting it will archive your case by default and you need to go over the whole process again.