My cat has Feline idiopathic cystitis and barely moves, how long will this sort of behavior last?

You should be able to cure this within a few to several days doing the following:

If the prescribed food is prescription diet dry food (vet recommended), stop it immediately. Provide high quality wet food only and give her for example boiled fish with some added water. Need to get water into her regularly and indefinitely.

Secondly, need to make the place less stressful for her. Incidentally female ginger tabbies are rare. If you are away all the time (understandable) it will cause stress - separation anxiety.

I am afraid you'll have to try and fix that problem (difficult). Idiopathic cystitis has 2 causes in my view: dry food + stress. Tackle both if you can. It'll work I feel pretty sure.

I see that you are tackling the stress element with a medication. Changing the environment is better and a permanent solution. Drugs aren't.

Vets tend to prescribe dry foods which are formulated to help cure cystitis but it is a misconceived policy as the cure is water to help flush the urinary tract system.

Dry foods cause mild dehydration which creates a good medium in which bacteria can develop in the bladder.

The above method should work within days.

Is she a full-time indoor cat left alone? If that is true, it is a sure-fire way to not solve the problem. Try some supervised outdoor activity. She needs to be active as well. Is she overweight? That won't help either. Sorry for being a bit tough. 😻😎

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P.S. please forgive the occasional typo. These articles are written at breakneck speed using Dragon Dictate. I have to prepare them in around 20 mins.