yeah, what the others say. I've currently clocked up six consecutive lesson cancellations, and the forecast suggests there will be more before I next get in the sky again! So frustrating. I was apparently close to solo at the end of my last lesson two-and-a-half weeks ago, but since it might be the week after next before I next fly, the delay has probably added an entire lesson to recover the skills I've lost.
When I was at Fly-in-Spain in Jerez, the lessons were notionally two hours, but I found that I reached the limit of my concentration at about 1h20, resulting in a maximum block time for a lesson of about 1h40. So the best bet is to organise two (or three if you're feeling brave!) lessons for a day each weekend, and assume that you've got at a 50-75% chance of it being randomly cancelled. Provided that the rest of your life is flexible enough to work around this, it should make the best use of your available flying time.
Best of luck!