How long does it take for CrystalsFLUTD in the cats to be healed/cleaned out?

That depends on what you mean by healed.

FLUTD (feline lower urinary tract disease) is typically a chronic condition caused by struvite crystals (more rarely, oxalate crystals) forming in the urine for poorly understood reasons. The pain associated with the syndrome is cause by these very jagged crystals irritating the lining of the bladder and urethra. Hematuria (blood in the urine) is quite common.

The usual treatment is a prescription diet designed to lower urine acidity to dissolve the crystals or a urine acidifier (which is less effective). We’ve had very good success with a diet distributed by Royal Canin called SO – it’s very effective and very palatable. The cat will often also be put on antibiotics, although this is usually a prophylactic treatment – bacteria is not usually found in the urine precipitate during a urinalysis. Pain medications and smooth muscle relaxers can also be used to reduce spasming and pain. There should be a gradual reduction in symptoms over a period of several days.

You should be very aware of the fact that your cat can still suffer a urinary obstruction even on treatment! Monitor his urination carefully. You can expect him to make more trips and spend more time in the litter box than usual because of the pain and irritation, but if he is not actually passing urine, you must get him to a veterinarian or emergency veterinarian immediately. Blocked cats will sometimes hide, often vocalize and cry out, and eventually begin vomiting and become moribund.

When a cat obstructs, a clump of crystals lodges in the tiny urethrra, and urine builds up in the bladder until it reaches the point that it can accept no more urine (imagine how uncomfortable it it is in the morning when you have to pee but you try to put it off – now add 12–24 hours to that). Waste product then begin to build up in the bloodstream. Urinary obstruction will cause death and it’s a very, very painful death. The faster you seek treatment, the less sick your cat will become (and the less expensive the resulting treatment).

Cats who develop struvitis tend to have recurring episodes. He may need to be on a prescription diet indefinitely. Some owners have had to resort to a surgery called a PU, where the penis is surgical removed and a “false” vagina is created to allow for a larger exit route for urine.

http://www.marvistavet.com/html/body_feline_lower_urinary_tract_dis.html
http://cats.about.com/od/lowerurinarytractdisease/Lower_Urinary_Tract_Disease_in_Cats.htm

Thanks! He has been at the vet all week (day and night), and they say he is ok, but he seems like he is still in pain.