Want to get automatic updates when a new post goes up, or a recent post changes? You probably want to subscribe to the Atom feed. You just need an Atom feed reader program or service that plays nice.
Suggested timing: if your feed reader sends conditional requests (most of them by far), then check at most once an hour. It'll work great.
If your "feed reader" is little more than a wget or curl that slurps down the entire feed every single time by not sending conditional requests, then check at most once per day. This minimizes the general wastes of bandwidth and CPU time spent on these redundant replies. If you want to poll more often than that, start sending conditional requests.
The canonical feed url (plug this into your feed reader) is:
https://rachelbythebay.com/w/atom.xml
Don't append ?null or other cruft to it. It accomplishes nothing. Also (I can't believe I have to say this), there are no spaces in that URL. If your feed reader is that braindead, consider using something else.
There's also a http version of the feed if you can't do https for some reason. It's almost the same thing, but the URLs inside are http instead of https. (Really, 'diff' them and see. That's the only change from one to the other.)
Go back to RFC 1945, aka the HTTP/1.0 spec. It's from May 1996. It specifies the "If-Modified-Since" header. All you have to do is set it in your HTTP request to the server using the exact data that was supplied in the "Last-Modified" header when you last got an update. Don't try to interpret it. Don't try to hard-code it. Just keep it around as an opaque token and pass it back.
Alternatively, you can store the value from the "ETag" header, and return it in an "If-None-Match" header in your request. This should also not be interpreted - it's an opaque token, and should be returned just like it was given to you. Bear in mind that if it looks like it's wrapped in quotes "like this", then those quotes are part of the value! (And no, I didn't have anything to do with it acting that way.)
If nothing has changed, you'll get a 304 status in the response and can know that you're as caught up as you can possibly be.
If your feed reader hits the server too often, expect to get 429 status responses back. This is automatic and happens right away. If that happens to you, cool out for a bit, fix your stuff, and try back later.
If you're getting 429s, you really should do something about it. Fix your feed reader or get another one. There are plenty of feed readers out there which play nice and don't squander resources.