Once a week's great if u can do that. However, its not really how much you play that counts but more what you do when u play. Also marshalling at a local site is a great idea... from the marshall's perspective you learn more in a day than you do in a week of playing (and the extra money contributes to your 'ballin funds). I used to run a site which meant I could marshal 1/2 times a week and train twice a week.... I think that helped me improve pretty quickly.
Kieran