Hi, August to October are the best months for the Mola-mola, it is believed that they are migrating for some reason, and are using the Penida reefs as cleaning stations. Depending on how your holiday programme looks like and if you prefer to stay close to the tourist facilities you have some options to dive Penida. Personally I would recommend to stay some days in either Lembongan or Candi Dasa, to dive from Padang Bai. If you stay on Lembongan, World Diving is the place to be.
Do realise that juli and August is peak season for divers, so expect crowds, espesially at Manta point and Crystal Bay.
I agree with Amanda that Tepekong and Mimpang are god alternatives with way less divers.
Also if you base yourself in Lembongan you can easier beat the crowds, keeping in mind that the tides around these sites need to be respected (sometimes mind-blowing currents!!)
And yes, of course there is never a guaranty seeing the illustrious beast.
You do know that there are many great world class dive sites around Bali apart from Penida. If you want a dive holiday with a sample of the riches of the diving on the different locations in Bali, the best way would be to do an extended diving safari. Maybe worthwhile looking into it?
Have fun and good luck with your quest for he Mola-Mola.