If I ETS in May of 2015, at what point am i eligible to reenlist?

Active or reserve?

I can give you the guard response. if your active then it may differ slightly but hopefully get you in the right direction.

If your are going to school you can extend at any time, regardless of your time remaining in service, to meet the time in service requirement for NCOES. My understanding is that if you are outside your re-enlistment window you will only be allowed to extend out to meet the requirement for your school.

quotas are set for each state for the national guard. If I understand it correctly I believe it refers to the anticipated need, which is set ahead of time. If your state did not anticipate a need and did not report that it would require seats then the school house cannot justify the amount of seats and classes to be conducted in the following years (ever wonder how they come up with the number of seats and classes for each year?). How far in advance they do this I am not sure about but what I do know is that the only way around that is as a stand by in a wait status.

You have to hope that someone doesn't show or is not able to attend whose seat is against another states quota, then they go down the wait list and pick up those waiting for a seat until the seats are full. exceptions beyond that have to go pretty high. 

If you were on the active side I would know the answer, but the reserves and guard are constantly changing their requirements and availabilities, so I am at a lost, but I would say that MSG Quick probably gave you the best resource to start.