The frequency of updates depends on what you're looking for. For example, I was looking for an iPhone 8 recently. I think there was one day in about two months when a bunch of models were listed; they sold out very quickly (and I didn't get one). Similarly, I was monitoring certain iMac 27" models, and there were a few months when I didn't see any new models meeting my specifications... yet just a few months before that, it seemed like every week or two there'd be a bunch that would hit the store.
To make your life easier, use
Refurb Tracker. I use it in RSS mode but if you don't use newsfeeds you can have it email you notifications, as well, although I don't know how well that works. You can specify which product(s) you're interested in, and in the filter you can do things to further narrow it down (for example, 512, 27 to limit to 27" iMacs with a 512 GB SSD). It doesn't ensure that you'll get to the products before someone else does, but it'll save you the time of checking the site yourself, and you'll be notified a lot quicker than if you were just checking it at random each day.