1 lesson per 1 or 2 weeks is sufficient to keep you in the rhythm. It's way better than doing a 4 week full time course.
Personally I would disagree with this, though if you DO go full time I would suggest that you do make sure you have some days off - say 1 day off for every 3 days on, or take weekends off, certainly learning in the UK you would be lucky to get 4 weeks good weather in one hit! Ideal would probably by fly 3-4 days a week, 2-3 hours each day, but most will either have the time and not the money or the money and not the time.