Limit on purge by URL?

I was trying to refresh myself on the limit of purge by URL requests per timeframe (per day, per month, etc) but have been unable to find the documentation. I was almost certain that it would appear at the bottom of the “Custom Purge” window, or in the API documentation.

The API documentation does detail a daily limit of 2000 cache-tag requests per day, but does not have any documentation for purge by URL limits.

Can anyone point me to the purge by URL limit?