Solved

[Kasteon]

Thanks for the replies.

[Seasend]

I don't believe there is any reference to mark your duration.
Maybe use a calender?

[Zoaic]

uh, either gaia cash purchases or order history idk it's in the little drop down where you'd see your account in the mygaia thing

[Nostalgic Sea]

Look at Gaia cash purchase history

It should say a date when you bought ___ item

Then calculate 30 days or one month after it and that's when Donor is gone,

[Piggyback-Chan]

Here

Got it from Myrielle xD