I'm one of those individuals who quarantine everything that is wet. I only have a hospital tank and use it for both quarantine and hospital treatments. It is usually up and running.
My standard quarantine period is 8 weeks for fish, and in that time I do a prophylactic treatment for ick for any tangs (3 weeks with
chloroquine phosphate -- I find this much easier on fish than copper or hypo, and it is also highly effective on treating marine velvet -- however, it can be tough to obtain -- it's a human malaria prescription drug), and a 3 hour praziquantel bath for all. If any diseases show up, then other treatment regimes are applied, and the quarantine period is extended.
I have a four foot (72 gallon) tank for this purpose, and use an Aquaclear 110 power filter for biological cycling. A second Aquaclear 110 is running at all times on the display system to keep a filter block seeded. My personal belief is that the biggest killer during quarantine is ammonia spikes, whether because the medication crashed the cycle, or the feeding and defecation rates exceeded the existing cycle's strength.
To counter this, I do a 33% water change every day until I see that the tank has been recycled, and add Amquel+ as needed to detoxify ammonia and nitrite when it's measurable. Through the years, I had a few heartbreaking losses
before I established this water change regime, sometimes in as short as two days of not keeping a close eye on the ammonia level.
For snails, live rock and such, I use a 4 week quarantine period, in a 10 gallon.