Come on, man... What sort of response are you expecting with a sentence like that?
Every strain is different, thus there is no straight answer to that question. Roughly 8 weeks. Perhaps if you included more info about your grow, people here could assess why your plant isn't flowering.
How long is 'this long'?? What strain are you growing? How long have you been flowering? What lighting are you using? Nutes? Temps? Humidity? Light leaks? .....