I get mine every other week, and I'm usually run down by the time it rolls around.
I used to have them three times a week and that was not even enough, because when I had to wait two days over the weekend, I had a bad crash and had to be taken to the ER.
The only thing I know that has made the difference, all things considered, is that I had Rituxan, and I think the residual effects of that has allowed me to go longer between treatments.