According to How API Keys Work (FAQ), the default daily API limit -- which is per site, per day -- is 300.

I started with 300, as expected, but over a period of days, I've noticed that the remaining quota has now been whittled down to about 70 something:

{
"items":
    [
        { "user_id":0000,"display_name":"abc","account_id":00000 }
    ],
    "quota_remaining":70,"quota_max":300,"has_more":false
};

Even after the UTC reset today, the quota remaining is still the same. Is this a one time thing where when I use all of this up, that's it, I'm done for good, or is this a bug?

What's more, this doesn't in fact seem to be counting per-site, as I can change the site query parameter to any site on the network, and the count still decreases, as if this is not in fact measured per site, but instead, network-wide:

https://api.stackexchange.com/2.1/users?order=desc&sort=reputation&inname=abc&site=serverfault&filter=!*MxJcsxUhQG*kL8D&callback=test

As an aside, I don't believe I'll need an API key, since the app I'm building makes requests on behalf of each individual user, and since I really don't picture ever hitting the 300 daily cap since I'm caching the data in chrome.storage.local.

This assumes, of course, that the 300 cap is in fact a daily cap and not a lifetime cap. Otherwise, my users would be able to use this for a limited time, until their own lifetime quotas have been exceeded.

Is this a bug, or am I going to need to use even heavier caching, like writing data to a file on the users' filesystems? (euuuuughh - shivers at the thought...)

Two days later, I checked my quota again by hitting the API. It now says 299 out of 300, so it looks like it's been reset. It's currently 7:38 AM UTC, so I'll wait until it rolls over again and do another test.