Fluid therapy can serve two functions: maintenance of hydration and diuresis.

Diuresis involves increasing the volume of urine by giving a surplus of fluids. This is the only way that BUN and phosphorus can be directly removed from the blood. (This is water diuresis which is different from the diuresis induced by drugs like lasix)

When a cat is hospitalized on IV fluids it's for the purpose of diuresis and the volume of fluids may typically be 300-400 ml per day.

The same effect can be achieved in almost all cases by giving similar amounts of LRS sub-q at home. It's actually much safer.

IV fluids go directly into the bloodstream and unless closely monitored they can lead to volume overload in kitties at risk for congestive heart failure (who typically would have pre-existing heart conditions).

Sub-q's enter the bloodstream very gradually. First they're absorbed into a sponge of subcutaneous fat (the interestitial matrix), then gradually into the lymphatic system. The lymphatics don't have an active "pump", the fluids are gradually coaxed along by the contraction of muscles during normal activity. Eventually they reach a duct in the neighborhood of the armpit where they may empty into the bloodstream. This is more or less passive, no pressure being involved, so there's no sudden increase in blood volume.
In short, sub-q's are self-regulating and very safe.

For most cats with kidney issues 100 ml /day of LRS is an average maintenance amount, i.e. enough to maintain hydration but not to provide diuresis to bring numbers down (often referred to as "flushing" the system).

Extremely high numbers are by definition acute and therefore potentially reversible with aggressive fluid treatment. They're not the result of chronic gradual deterioration.

I've had nine cats who grew old with me and inevitably several of them had failing kidneys, at least half of them having through-the-roof renal values at diagnosis. All were able to be stabilized with aggressive at-home treatment, none were ever hospitalized for IV's.

At Coco's worst crash her renal values were:

After about six weeks at 200+ ml/day and a lot of care Coco was once again bounding through the house for almost 3 more years. Her renal values were still stable when she was eventually overwhelmed by other issues.

NOTE: The assumption is that lactated Ringer's is being given. Under no circumstances should kidney kitties be given 0.9% saline, though this sometimes happens in ER's. If a kitty has been put on saline it should be stopped and immediately replaced with LRS!

Please see Ringer's vs Saline