Basically, I launch my Slime/SBCL and do my development by editing and evaluating code in my Emacs buffers. This updates the running image and I can check the results interactively. I regularly save my buffers as one does.

This process can continue for days.

There are times when I need to relaunch my Slime/SBCL and start fresh. These are usually cases where I do some refactor that eliminates some functions, or when a bug causes crashes and I feel it would be cleaner to start fresh.

All pretty common sense.

I never save my Lisp image and relaunch it. Maybe I would if I was delivering binaries, but my project is open source.