Someone who wants to release may wait a while but if you are going to "top lease" someone elses lease near the end of the term you are making a bet that the existing lease will expire. If they know for a fact or near fact that a company will not or is not likley to exercise its options within the lease or to drill, then a new party may take the chance that it won't get drilled before May.
Since applications have to be made early enough to get permits, time is running out for leases that are due to expire in May.
Some companies have anti-top lease provisions in their original documents and I would not sign those.