I have had an attorney cite a reasonable time to wai at 10 dyas, with warnings that the deadline is closing in. If not met you enforce the policy. If they don;t send back the lease in the timeframe, you bump it to month to month - the NAA lease template generally allows, but check your state - and they are charged the M-M rate. The animal that needs the vet statement can't live ther without the statement and needs to go - if you have already stated that. Obviously, check with counsel. I am NOT counsel, just experienced. They are managing YOU, You need to manage THEM. Just do it!