We spent 8 days (Sun-Mon) at Pearl this past October. It was also our first visit.
The weather was fine. There was one day where it was cloudy and a bit cool to be in the pool so we just moved over to the jacuzzi.

As others have said, it is hurricane season so I recommend decent travel insurance.
We arrived on Sunday and departed on a Monday. The extra hotel night was free due to the 4th night free and we ended up saving a couple bucks on airfare. I wouldn't worry too much about arrival dates affecting your ability to meet people. We found the crowd swells around the weekend but there are always nice people to socialize with whether or not you are into swinging. Just head over to the pool bar. You'll find folks that have been there for a few days will recognize you're a fresh face and chat you up.
As far as length of trip goes, 8 days is probably the limit for us. We were glad to have the 8th day of vacation but we were ready to return home at the end of the trip. Our days seemed to go something like this Wake Up->Morning Sex->Breakfast->Shower->Pool->Lunch->Pool->Jacuzzi->Shower->Dinner->Club->Jacuzzi->Sex on Jacuzzi Bed->Sleep. It was nice to break up the trip with a couple of adventures so we jumped off the resort twice, once for the Cancun Boobs cruise that leaves from Temptations. Second time was for lunch in Puerto Morelos. The couple we did the boobs cruise with spent a day snorkeling at Xel Ha. We regret not doing so with them.
Hope that helps.