How frequently can a cat get crystals?

My question is, how soon after a cat has been rid of crystals can they get them again? And how frequently can this happen?

It can happen continuously.  (Been there, done that...for years...)

You may want to specify your question a bit and you'll get more specific and helpful responses rather than my stoopid one liner above 

OK, so let's look at what causes crystals. I'm assuming you mean struvite crystals, yes?

Looking at it from a chemistry view point struvite crystal formation depends on 3 factors.

The pH of the urine. Struvite crystals form in alkaline urine meaning as pH of less than 7. A species appropriate diet (meat based without veggies and grains) will naturally result in an acid urine where struvite crystals CAN NOT form.

The concentration of the urine. The more concentrated the urine is the more likely that struvite crystals will precipitate and cause a problem. Even if a urine is very alkaline, if it is dilute enough, crystals are unable to form.

Time. The longer concentrated, alkaline urine sits in the bladder, the more the risk of crystals and stone.

The first item, the pH of the urine is hugely dependent on diet!  Meat, bones and organs, the diet cats evolved on for millions of years, produce an acid urine.  Acid urine=no struvite crystals.  Do not feed foods with grains and veggies!!

Concentration of the urine is related to the amount of moisture in the food. Cats, being desert animals have a very poor thirst instinct. In their natural environment just about ALL their fluid intake is from the fresh kill they are enjoying.  Cats on a kibble diet are chronically dehydrated.  This is not good, not only for the sake of crystals.

Time. Again, here is the fluid intake. A cat who is dehydrated will urinate a lot less frequently. The urine will stay in the bladder for a long time, and if that urine is from a cat eating a diet with grains and veggie, that urine will likely be more alkaline and that n combination with the time in the bladder, and there s a breeding ground for crystals.

Basically, feed an acidifying diet which contain the necessary amount of moisture. This alone will take care of a LOT of urinary issues with cats.

Edited to add:

Diet and fluid intake is one aspect, but not the only one. Stress plays a big role. (Not what we think is stressful, but what Kitty feels to be stressful.) And also, outside cats have a LOT lower frequency of struvite crystals. No. I'm not advocating for kitty to run outside. But it tells us we need to provide, somehow, inside, what kitty would be enjoying outside.

thank you for your response. your post is very informative and written very well. 

i have learned a lot since browsing through this site. what is troubling me is that i am afraid that my cat could get these struvite crystals again if i dont feed him the correct food.  the problem is that my cat was put on a prescription diet and he was unable to tolerate the food so i am trying to figure out which to put him on now. he has a sensitive stomach to a lot of foods so i am trying to find one that he will no throw up and that is good for him. so i have been looking into different foods and i am thinking of trying Purina Pro Plan Focus Adult Urinary Tract Health Formula Chicken Entree in Gravy Canned Cat Food. I did just put up a new post asking if anyone has used this food and if it seems to have worked out for their cats. he used to eat purina foods and was usually okay on them and didnt throw them up. so i would like to try this one but i am not sure that it will be good for him. i really dont want him to get them again if it could be prevented. i wish the prescription food would have worked out for him.