I'm not living in Monterey anymore so I don't know the weather conditions and therefore can't give a good guess. It can be a few days, to a week, to a month.
The short answer is blooms depends on weather a lot. And like weather, it is a best guess to determine how long a bloom will last. The Jellyfish repellent can be a hit or miss, it is not a sure prevention from a sting and it isn't really tested under diving situations, only beach bathers.
Long answer for the bloom, a bloom will develop and last as long as there is a heavy saturation of food source for the jellyfish, that it primarily plankton and bait fish. What causes a bloom, to dumb it down is rainfall and wind primarily (at least for Monterey, CA area).
Rainfall on land creates a lot of runoff of fertilizer, pet poop, bird poop, and the like which adds lots of nitrogen, phosphate, and lovely nutrients for plankton & algae. That can help boost bait fish, but regardless lots of salps and jellys will bloom with a heavy increase of plankton and algae blooms alone. Or you can just get red tide and wipe "everything" out for a while.
The bloom will die a little after they overconsume the food source.
Wind can also create blooms because it creates upwelling which brings deep nutrients to the surface near shore, which creates the same situation as rain runoff. Summer time usually brings lots of wind and upwelling to the Bay Area and Monterey.
This is where popping into Breakwater Scuba and speaking to one of the counter guys will help. Lots of their staff right now are marine bio grads from UCSC & CSUMB. Sean, Maggie, or Leroy can all help you with a decent guess. But it is just that, a best educated guess.
For the jellyfish repellent. It works by one mechanisms. Making your skin slick which can prevent jellyfish tentacles from sticking and thereby creating less pressure on the testicles, which means they're less likely to sting you.
You can do the same exact thing more effectively by smearing yourself heavily in Vaseline. But don't do that.
Vaseline ruins silicone and scuba gear because it's a petroleum product and degrades rubber and silicone. It also ruins your mask seal to your face.
The jellyfish repellent won't do this but it can ruin your mask seal.
Now with the repellent, this is where diving runs into a problem. Your neoprene can house a jellyfish stinger and you can then get stung taking off your wetsuit.
Jellyfish stingers are called nematocysts and tentacles are covered in thousands of them.
Nematocysts fire with any disturbance (ie pressure), and it is a passive thing independent of the jellyfish's "thoughts" if you can call it that. You can still get stung by a dislodged tentacle or a dead jellyfish. You can't see nematocysts and they can deposit from tentacles onto your skin and fire latter on when disturbed. Luckily they're all single fire and done, which comes into play if you're strategic.
Freshwater fires nematocysts. Distilled White Vinegar neutralizes them from firing. Salt water may or may not fire them. Hot water (or heat) cooks the poison and cures continued pain in your affected area if already stung.
So for desuiting after a jellyfish dive you'll want scrub your gloves together over fresh water to fire off any nematocysts on them. They can't go through your neoprene. Then shower under fresh water fully before desuiting. If you have vinegar handy, gently rubbing a soaked cloth/glove on your face will scrape off any nematocysts without firing them.
With vinegar or fresh water, it only has to touch the nematocyst for the second to get the reaction. So you don't really need to spend minutes in "decontamination".
If you are stung, immerse the area in hot water, apply a heat pack/warm sand, or lay on the warm hood of your car until pain stops increasing. (not scalding, not edge of your comfort hot, just hot). After pain stops increasing, you can treat as you like. Cold pack for swelling, lotion for chaffing, puppy for hugging and emotional support......
For diving in jellyfish a decent option is to try to avoid swimming into the bloom and go over or under it. Most of my experience you had that choice if you went off your usual site landmarks. If the jellys are at your entry point, just cancel the dive. If you find a bloom, navigate to avoid them or treat it like a boat lane and navigate under/over and plan to navigate properly back so you don't have to surface in them.
