If a loved partner or spouse enters rehab, would you not want to be with them anymore? Would you stay if they relapsed months or years later?

One thing at a time. You better believe I'd stick with anyone who wanted to better themselves. Like glue. They'd have every bit of my support, and we'd take it one day at a time. And if they fell again, I'd tell them, "That's okay. Let's get back in the boat and start paddling again. We won't be defeated!"

if they were willing to go to rehab in the first place, that's a good thing right? Not sure why I'd leave them at that point as opposed to some point before they agreed to do it.

I would do my best to be supportive.

I would be there, forever, and be supportive.