We considered off plan in Turkey in October, but changed our minds. However that's what we were told, but I think it was 10% then almost immediately 40% then 40nearing completion date, with 10% held back until everything was satisfactory.

Dont pay the final installment until the place is finished. you then have to get your ruhsat and tapu transfer which takes ages. make sure you have a good legal contract that you understand and is notorised in English. a lot of people but off plan successfully so good luck

It is usual to hold back about 10% until the property is complete and you have seen everything and are satisfied with it all.
If you are buying off-plan, ask at contract stage for a clause to be put in the contract, for example, if the property is not completed in time and you wanted to rent it out, the builder has to pay you the rental money you would lose if the property was not ready.
If you need any more info about buying off-plan please email me.