After a Eurasian patent is issued, it is valid in all member-states of the Eurasian Patent Convention till the next date on which the maintenance fees have to be paid.
This date corresponds to the date of filing the Eurasian patent application or the International Filing Date - for PCT applications, transferred to the Regional Phase in EAPO.
Before that date the patent holder has to elect the countries for further maintaining the patent and pay the fees for all countries elected.
At the time of payment of fees for the elected countries it would be necessary to pay for the past years of patent validity and also for the next year. Usually the patent is granted subsequent four years from the International Filing Date. So, as of the next date of payment of the maintenance fee it is usually necessary to pay the annuities for the next (fifth) year of patent validity and the passed four years starting from the International Filing Date as well.
According to the Statute of Fees of the Eurasian Patent Office, the total sum of the annuities for the first five years of patent validity for all countries makes up $3185.
Usually a Eurasian patent is issued about half a year after the EAPO receives the Request for grant of patent and the official fee for the action. So, if you receive the decision of grant of patent in February and the International Filing Date of the application is in October, it would be rather critical to choose the date of filing the Request for grant of Eurasian patent.
In case you file Request soon after filing, for example, early in March, the patent could issue before the date of payment the annuities in October, and you will have to bear these costs the same year.
But if you file the Request in May just before the deadline, the patent could hardly be expected to issue before October, it will issue later. Thus, you will have the whole year for maintaining patent without further costs and there will be enough time to select the countries for further maintaining the patent and develop the best patent policy.