How long would my luteal phase be if my menstrual cycle happens every 25 days every month?

Depends on ovulation. The luteal phase starts after ovulation and is 14 days by the textbook. To determine the length, you need to accurately know the day of ovulation. You should check for ovulation with urine tests or bbt. It is possible that you have a normal luteal length but short follicular phase. If you have a normal follicular phase (textbook = 14 days) then the luteal phase is 11 days. Best wishes.