when do you end a fun relationship that you don't see a future for?

A couple of months ago I had a relationship end (due to a move) with someone that despite our ups and downs I could completely have seen myself marrying. This kind of relationship has been the norm for me for essentially my entire dating life (3 long term relationships form 80% of my relationships, a few 2-6 month flings in between)

I wound up finding out that someone I knew through work (different dept, we'd hardly spoken beyond some basic pleasantries) had an interest in going on a date with me whenever I was over my breakup. Not long thereafter I found myself enjoying her company and things developed into a fun relationship. I have fun with her and genuinely enjoy spending time with her etc. I even have some pretty strong feelings for her so it isn't like she's just a FWB.

The last few weeks I've realized that as much as I like her, there are quite a few things that keep me from thinking of her as someone I'd settle down with.

If you were in her situation when would you want to know that I felt this way? Since nothing specific has been brought up do I just wait and break it to her if/when it does? Also assuming I should say something, any tips on the best way to say "I like you, I'd be happy to keep dating you, but I'm not in this for the long term"

(FWIW, I do care about her, and that is why I want to handle this as delicately as possible.)