When Wargaming game servers are down for maintenance for a long time, all players with Premium Account, or other temporary purchased services active at the time that maintenance commences, are compensated in the form of Premium Account time and other services.
The duration of such compensation is negotiated on a case-by-case basis and depends on the duration of the maintenance.
Compensation of Premium Account time and other temporary purchased services starts from the moment that maintenance commences. Thus, the Premium Account time and other services you purchased are preserved.
The following news item is posted on the official game website:
"On January 12, from 3:00 to 12:00 (UTC) EU game server will be down for maintenance." Players, who have a Premium account or other temporary purchased services active at the time that maintenance commences, will be compensated with one day of Premium Account day and other services starting from January 12, 3:00 (UTC)".
Let's say you bought Premium Account time on January 11 at 15:00 (hereinafter UTC), thus, at the time of commencement of maintenance (January 12, 3:00) 12 hours of your Premium Account will have elapsed.
On January 12 at 3:00, one day of Premium Account time compensation will start, expiring on January 13 at 3:00.
Thus, on January 13 at 3:00 your remaining 12 hours will start, expiring on January 13 at 15:00.
Compensation for other temporary purchased services (such as camouflage) works similarly.