Since 2010, Turkish visit visas issued at arrival ports state that they are valid for multiple stays up to a maximum of 90 days in a 180 day period.
Up to 31 January, this rule has not been enforced by the Turkish immigration authorities, who were operating a flexible interpretation of the requirement.
However, from 1 February, the new visa rules will be fully enforced, and you will only be able to stay in Turkey for a total of 90 days in every 180.
If you plan to remain in Turkey for a period of more than 90 days after 1 February, you should either enquire about longer stay visas from your nearest Turkish Embassy before departure, or if in Turkey, you should enquire at your local Foreigners Police Department (Yabancilar Sube) before 90 days expires. If you stay in Turkey for long periods of time, it may be worth considering obtaining a residence permit.
Be careful not to overstay the 90 day period. Overstaying is taken very seriously by the Turkish Authorities. Those who overstay the 90 day limit could be fined, detained, deported and banned from re-entering Turkey.