Technically you only have to top up a Turkcell prepay sim every 270 days for the line to remain active for incoming calls.
In the old days it was 90 days, but now it has to be topped up every 180 days to keep the line active for outbound and inbound calls.
From day 181 it goes into a 90 day Semi-Active period when it will only be able to receive incoming calls, but on day 271 it will be disconnected. You will also be unable to keep that number as it goes back into the Turkcell system.