Our 'Balance' endpoint will output the number of requests you have for the day. If you're on a custom plan, it'll output the number of requests you have left until your requests reset.
Example:
https://us1.locationiq.com/v1/balance.php?key=YOUR_ACCESS_TOKEN&format=json
The output looks something like this:
{"status":"ok","balance":{"day":4996,"bonus":0}}