Hola Alex
The question ¿Desde hace cuánto tiempo? is one way to ask "For how long?"
But you could always ask this way as well "¿Desde cuándo? (Since when?) or even use "¿Hace cuánto tiempo?(How long ago?/For how long?) These lessons will help you understand it better:
desde (hace) with the present tense
hace + length of time + que with the present tense
hace + length of time + que with the simple past tense
Saludos cordiales
