Her request seems a little...personal. I wouldn't ask a neighbor for help with housecleaning. If she's not clearly disabled or whatever, the next time she asks, I'd say: "Sally, I noticed you ask for housecleaning help pretty frequently. You know, I'm glad we're neighbors and each other's contacts, but I don't have time to do housecleaning other than my own. But I'm concerned about you: Are you having some physical issues or other things that are keeping you from being able to do it? Do you need a ride to a doctor or anything, if that's what's up? I want to help as a friend if you have a problem that means you can't do it."
That makes you the good guy (you DO want to help if she IS truly having a problem you really may not know about, right?). If there is something actually wrong, better to know about it and give her the benefit of the doubt.
AND if this is really just a matter of her being fully capable but just whining about her housekeeping -- asking her sincerely if she is ill or has problems should (one hopes) shame her enough to make her stop asking.
One other possibility here. She may be simply lonely. If she's mostly asking for your company while she cleans, she may be wanting the company, not the cleaning. She may be socially inexperienced or inept and may not know how to say "I'd like to get together as friends and do something." She may be in financial straits so she can't say "Let's go out" and she doesn't know what else to say except "Keep me company while I....clean." So to be a little gentler, I'd say, consider the possibility that this isn't about cleaning at all but is about her wanting another adult to talk with, especially if she's home alone most of the time. So you also could ask her up front: "Hey, is this really about housecleaning, or do you just want company?" And maybe ask her to go out for coffee for an hour. She could turn into a great neighbor if you get past the housekeeping thing.