Most women in May’s situation return in the first 60 days, but the first month is the most likely.
Quick return time stats tend to apply to DV or super-volatile situations. Living with a pilled-out, drunk rage pig is admittedly inherently volatile, but it's a little different than a battery situation [yes, I am assuming that the abuse here was primarily verbal and emotional (possibly financial) and I'm not going down any other speculative paths]. That said :
At this juncture the only reason a woman returns is if their new situation is so dire (like destitute in shelter with a baby, or extremely isolated ) that return to relationship seems an improvement over their current situation.
Lack or fear of lack of resources is one reason, and another can be that the woman didn't cease contact and is being reeled back in emotionally (or in some cases threatened). Manipulative exes will find any crack to try to seep in, and women with self esteem issues fall for it way too often, especially though not only when resources alone are dire.
On May - I can't quite tell if she is a sap or a mercenary. Are her motivators desperation for love/romance, or being on the arm of a known guy (even in a micro-community)/having clout/$$$? I'm leaning to the latter if she's not bent yet (though something Harry said made me think there might have been some wavering).
If a sap, if in contact or following events, she could fall prey to emotional appeals if they were to see each other (believing miraculous sobriety and "our family" and "i love you" "I'm begging"). If a mercenary, I wonder if the events of the last week would have her feeling stars in her eyes again. Hopefully she has enough distance now not to look back. Ralph sounded a little more doubtful about those prospects yesterday.
But I think the best thing that happened here for May, in addition to staying at least apparently offline, is that she left and went straight to family. Not only is there no fear of landing in a worst/more precarious position by leaving Ralph, as you mentioned, but she has much more. Whether she's with Rozy or not, it has seemed they were both scooped up and taken in with joy and support. Whatever her feelings about home/family have been in the last few years, there is comfort and security there: there are people to take care of Rozy (and probably teach May some things); there's a bed with clean sheets and a refrigerator of unspoiled food and no filth and bottles rolling around the floor. She's earning a bit of money of her own with no one screaming at her about it, and without it mysteriously disappearing. No one is yelling and cursing at her then passing out sitting up; the baby isn't screaming; the electricity isn't getting shut off; she's not being humiliated on camera. In addition to feeling relief and feeling good, and being around (I assume) stable and functional people who are nice to each other and to her, the contrast between a presumably healthy and comfortable environment with how she has been living should surely make an impact. When you've been on a downward slide over a period of time, accepting worse and worse, you often don't even realize how far away from normal you've fallen. Spending time in normal living standards for awhile should let her see how degraded her life has become. Hopefully, the rewards of a duller, saner life outweigh the allure of playing house in squalor.