>>57305300E-sekai donothon start dates:
yuko - 30th Jan
rin - 4th Feb
juna - 26th Feb
pochi - 4th Mar
fuyo - 16th Mar
Fuyo and Rin both had their donothon on their birthdays so I would expect the same for Kai (13th Sept) and Momo (8th Oct) which would likely make Momo's donothon either last or second to last. 
Unless they're really cramming them together then Kai should be second cos of the birthday thing so if Poko's is third you have until mid September to save up at least or more time if she's 4th/5th.
I'd also guess that all of the donothons will be wrapped up before the end of October.