I was there this past June 19th. They were open for 2 hrs I believe, 5:30 to 7:30, although the track closes 15 minutes before closing time. The threat of rain was my biggest worry, and it was in the forecast, heavy at that. Luckily, the rain didn't start until just after the track closed.
I purchased a card with 4 laps on it. 95 Euros i believe. I figured if I wanted more, I could get more. I waited almost 15 minutes after they opened to take to the track. There wasn't much traffic at all. It may have been because rain was in the forecast, I don't know. I was there because I had picked up my new M3 a few weeks before. I had already had the breaking service done in southern France.
My first lap was absolutely fantastic. I pushed the car as hard as I could see. I got back to the paddock area, jumped out of my car, and jumped around like a rabbit. Needless to say, I had been dreaming about this for a very long time, and it went far beyond my wildest expectations.
I let the car cool down about 10 minutes and went back out. Each lap was exciting, but not nearly as exciting as that first lap was, but they were all fantastic. I had enough time to do a couple more, but I had had enough. I needed to catch my breath. Way too much excitement to pack into a 2 hour timeframe.
So, the weather it seems to me is your biggest worry. If it's not raining you will have a blast.