Plumbing – How much length of pipe is needed to use a ‘donut’ type PVC to iron pipe connection

85 years old. The biggest problem is that cast iron is heavy. By removing a section of the piping you might end up inadvertently messing up the mounts for the old pipe. If it is at all possible I would replace it with PVC, otherwise, I would leave it alone.

You've already accepted, but to properly fix this:

I'll assume it's all polyethylene pipe (typically used for well piping). The fitting also looks like nylon, which in my experience is easy to damage and often leaks. I much prefer PVC (which is typically gray) or brass.

Take everything apart, remove the elbow, and throw it away. Heat the end of the poly pipe with a torch, both around the outside, and a tiny bit on the inside. You want it to be warm, but not melted/squishy/burning. Put on two clamps, then put the fitting in, and then tighten down the clamps (about 1/4" apart, give or take). Repeat with the other end.

If you've never done this, you may want to practice on some scrap pipe first, especially if you don't have a lot of extra to work with. If you do melt the pipe, you need to cut the end back and remove the melted piece.