The club I went to had yoga classes, forget which form it was.
But I went once a week and it wasn't much help, also I was lifting heavy weights so that also didn't help with flexibility.
I would say 2 times a week minimum, anything less is very slow in making progress.
About my class:
I went to the back of the class for my first time because I knew how inflexible I was and didn't want to be looked at.
However as we went through the forms I found out the back of the class turns into the front of the class.