im a white belt and i go for 2 hours every saturday. I only want to become a yellow belt so that i can start sparring. It looks fun and its boring to sit around for 15 mins watching the higher ranks spar.
It is different depending on the schools /style. Generally it takes 2-4 months in Karate and Taekwondo between ranks and so that would be 2 - 4 months if they don't use stripes and 4-8 months if they use stripes and Yellow is the next belt. Good Luck!