Recreation.gov books on a rolling window roughly six months deep. Each day, one more day's worth of inventory becomes reservable, and for most campgrounds that inventory appears at 4:00 PM Eastern. Some campgrounds instead release in monthly blocks on a fixed day — Upper Pines in Yosemite, for example, opens on the 15th of each month. The campground's own page on Recreation.gov states which applies, and Kestrel shows it on each campground page under the booking rules.
For a genuinely popular campground, the drop is effectively the whole contest. Sites can be gone within a minute. Nothing about an alerting service changes that — if you want a Yosemite Valley weekend six months out, you need to be at the keyboard at 4:00 PM ET with your dates already chosen and payment details saved.
Where alerts do change the outcome is everything after the drop. Once a campground is sold out, its remaining inventory comes back only through cancellations, which arrive unannounced. Kestrel rescans every two minutes and every 30 seconds around the daily release window, so a site that reappears at 11 PM on a Tuesday reaches you rather than going unnoticed.
A practical approach for high-demand parks: watch several campgrounds in the same area rather than one, and keep the minimum-nights requirement honest. A two-night alert will match far more openings than a four-night alert, and the difference is usually the trip happening or not.