I do 50 sit ups per morning and night. at this rate how long will it take me to get a abs?

Get trim also: Exercise several times a day using whatever exercise it takes to give your abs a strong burn. You must also trim down, losing almost all your body fat, dieting and doing cardio. Speed depends on how hard you diet / do cardio. This is very worthwhile and i hope that you find fitness far more rewarding in addition to your having a great body to show. Best wishes.