I've been there twice in two separate years in July and August and we had Mantas both times. Unfortunately that's not much of a statistic. I dove with Kona Honu Divers and they didn't seem to concerned about them not being there, at least at that time of the year.
By the way I would recommend Kona Honu over Jack's, it was a bigger boat, less crowded, and you can park practically right next to the boat. I saw a bunch of people at Jack's cramming into a van, for what looked like a ride to the boat.
Kona Honu has a great operation, however make sure you talk to Maggie, Glenn tends to forget things. But he and his crew treat you real well, and I short of an Aggressor I haven't found a better dive boat.
By the way, even my daughter (now 15) was impressed with it. And quite frankly its hard to impress a teenager. Its been called one of the 10 best dives, and so far I haven't found anything that comes close.
One suggestion, take a 3 mm wet suit, you sit still for quite a while and it gets chilly, my wife and daughter both had to bail early because they got cold, and even I was starting to fill it a little.
Bob