As a fosterer and a foster coordinator, my dream would be to have someone willing to be a respite fosterer for cats who have have overstayed their welcome with a fosterer. We honestly don't always know how long an animal will need to be in foster (and I feel that we DO tell people that as best we can but we also need to be clear with people that the stay MIGHT be shorter than they are expecting... and I think those 2 can get muddled in people's mind). I absolutely understand that sometimes people need a break from fostering.
Here's my example: Today I said goodbye to a foster that came to me when I was only a fosterer. She was with me for 800+ days. She was supposed to be coming for a 'break' from the adoption center but every time she went back she reverted to the same behavior. (which was aggressive and which scared volunteers.... and we like to keep those volunteers coming back.) The adoption center manager simply did not know that that's what would happen. Fortunately she was an easy cat in a home setting and I always knew that if I truly needed her out of my house my director would work with me to make that happen.
Maybe what we need to do is to give the more predictable fosters to those who absolutely need to know the timeline and save the less predictable ones for people with more flexibility? As a foster coordinator, I know we don't give people dates for when their cat will be out of foster because we just don't know when there will be space in our adoption center. However, it DOES become obvious when a cat is going to be long stay (I've had 3 with me for over 300 days) and I think it's important for there to be an honest dialogue between fosterer and foster coordinator or director about how to proceed.
The other issue that comes up is behavior/medical concerns/etc. Sometimes what looks like a minor issue becomes chronic and extends time in foster. That's another situation where I think there needs to be very open and honest communication about what a foster can and can't do.
So much of this is about communication. I think we all really need to be heard.