Well, a lot of factors go into determining a timeline, especially when you're dealing with a cold-blooded animal since their metabolisms work differently than ours. Warm temperatures, pristine water quality, a quality high protein diet, and to a point, genetics, will all contribute to his speed of recovery.
As for his fins growing back, since the dorsal is completely gone, it probably won't come in very quickly or very well--if it comes back you can expect it to be twisted, ruffled, or misshapen. As for the other fins, luckily, your guy is a veil tail, so the fin should come in fairly straight and more quickly than tail types with lots of more delicate branching like halfmoons or deltas. This will depend on heat, diet, genetics, and sanitation just like everything else.
As far as if he will recover or not, you'll probably be able to tell best by his behavior. If he shows interest in food and goes after it, he is expressing his will to live. If he is alert and reacts to his environment, his outlook is even better. If he is listless and shows no interest in food, his prognosis would be quite guarded..
As far as the instruction you've received from the pet store--as I mentioned earlier, I don't have any personal experience with treating infected ulcers so I'm really just going off of reading I've done from other sources. However, the experiences I've had at pet stores lead me to believe that they typically don't offer accurate information. Many employees have been given training that focuses on making the store money rather than giving you the proper tools to help your fish live happily, so take what they say with a grain of salt and always rely on the power of your own research.
Leaving in the methylene blue is definitely a good idea for a fish with an open wound like yours has--the bottle should give you dosing instructions depending on what other ingredients might be in it. Some methylene blue comes with salt added, so if you're salting the water anyway, make sure you account for that.