I will preface this by saying I am a veterinarian that exclusively treats felines. Below is some of my general advice since I do not know your specific situation. I am presuming that a urinalysis has been done confirming struvite crystals. This is also advice for a young male with no other medical problems. I normally don't post regarding veterinary medicine on here (or much at all, really), however, I have had a few clients in the interest of saving money on food loose their pets when it could have been prevented.
Struvite crystals in the urine are relatively common in cats, both male and female. The crystals can coalesce to form bladder stones. Male cats are at much higher risk of developing a urinary obstruction which can be fatal than females since their urethra is much smaller in diameter. An obstruction can be caused by either small stone or (and this is important) clusters of crystals. If your male cat is showing clinical signs of discomfort while urinating than he is at risk of becoming obstructed.
We do not know exactly why certain individuals are more prone to developing crystals than others. A 100 cats could be eating the same diet as yours was previously and only a few would get crystals. The prescription diet you were given is formulated to both dissolve crystals that are currently in the bladder and to prevent the formation of new ones. It is very important that your cat stay on a prescription diet until a urinalysis shows evidence of no crystals. (I usually recommend a urinalysis one month post a diet change). The cost of food is nothing compared to the cost of treating a urinary obstruction or surgery to remove bladder stones.
For long term prevention of crystals:
1. The prescription diets work extremely well. The number of urinary obstructions and cystotomies have dropped dramatically since the availability of these diets. Perineal urethrostomies (basically a penis amputation) are almost unheard of nowadays. The three major diets are Hill's c/d, Purina UR and Royal Canin S/O. They all do the same thing and it's just a matter of taste preference. They cost about the same. Many of my clients use Chewy.com to order food. The large bags (16-17#) are more economical than the small ones. If one of my patients has an obstruction or surgery, I generally recommend they stay on the prescription diet. They are often happy to do that after going through the trauma and cost of those treatments.
2. There is only one over-the-counter diet that claims to prevent struvite crystals (if your cat has another type of crystal this won't work) - Purina for Urinary Tract Health. This works well for some in the prevention (not dissolution!) of new crystals but not all. Once you have a clean urine and absolutely no clinical signs, this diet can be tried. The urine must be tested again after any diet change. I can not stress this enough - it does not work for every cat!
3. In theory, just because one OTC diet caused crystals in your cat doesn't mean any of the hundreds other diets out on the market will do the same. Then again, maybe they will. Also in theory, being on a canned only non-prescription diet prevents stone formation (but not necessarily crystal formation) since being well hydrated flushes out the bladder better. Again, in those individuals that tend to form crystals, a urine should be checked after every diet change. For me, compliance regarding urine checks is a big issue and then I see the cat again for the same problem that started it all in the beginning.
As for your last question regarding the length of clinical signs, it depends. If there are just a few crystals and no other complications such as an infection, stones or lots of crystals, most cats are symptom free with a few days, sooner if coupled with pain medication and canned food only. If I have a patient that is not completely normal within 72 hours or at most a week after pain meds, diet, increased water and sometimes a drug to relax the urethra, I recommend some type of imaging of the bladder to rule-out stones (needs surgery) or large amounts of crystals (for which there is another specialized diet to use in these cases). If symptoms are still present, you need to call your veterinarian for advice right away.
No matter what I write here, I can't actually DO anything to help you, so, please, please, please call your veterinarian for advice ASAP if your cat is not 100% back to normal and make sure you go to your recheck. The internet is not always a good place to get advice (even here on MMM!).