you can renew the donation or subscription (same thing) with 90 days still on there (probably even more) and even 2 months after it has expired so you dont have to wait for the mac to clear, if you usually buy new donations and forget to switch over. this can be very annoying if using a mag or avov.
according to iks66 as it says on the renew page "To be able to renew a Route it shouldn't have more than 3 months as remaining time, it shouldn't have more than 2 month that expired and it shouldn't be a Trial." i assume the same goes for rocket or it might be 30days both ways as someone else suggest.
you should really treat your donation as a subscription keep your donation number tight and renew it just like any other subscription. imho buying a new sub. is crazy, you dont buy a new cable, Netflix, amazon, or whatever subscription each month/year either it is usually automatically renewed, and some you renew yourself like real debrid or others its the same thing.
especially if some say they need to catch up sometimes, why give them more work.
or why give yourself more work switching mac addresses around, wait until they expire so you do not loose days etc. seems crazy to me, as said you do not do that anywhere else either.
I have renewed myself and friends even more times with lots of days remaining (the new ones will just be added) one also after he expired and never had any issues. but that's just my 2 cents.