Thanks @manu for your reply. I just called them today, the time limit is indeed 90 days, you have to come home at least once, and it will start from 0 again.
Another reason, which is probably bigger than the one you mentioned: It does not cover business trips or anything related to your work. It's only meant to be used for vacation.
About "they try to avoid payment": That's the case with every insure. Actually, it's just a regular insurance provider, which is used by the banks providing the credit cards.