I've been flying once a week on & off with 1-2 months gaps occasionally (due to holidays/uni exams/weather). About 55 hours TT (since June 2008) and Im still working on the 2nd leg of my X-country. From Dec-Feb the weather slowed me down big time. Not the most most effective way of learning but other commitments can slow you down.
If you have time and the money 2 per week should get you through steadily, and make the most of this summer.