Calendar usage limits
Occasionally a user may see an error message that he or she cannot perform an operation due to Calendar limits, such as:
Unable to save event. This means you have reached the limit for the number of events you can edit.
Calendar usage limits exceeded. This is the result of an API call. (Don't mix this up with the message "Daily quota exceeded," which points to insufficient API quota.)
There was a problem saving your changes. Please try again in a few minutes. This message will be followed by a more descriptive description on the limits that have been reached.
Why is this happening? Google Calendar puts certain limits in place to protect our users and infrastructure from abusive behavior. When these limits are reached by a user, Google Calendar will go into read-only mode for that user, and all edit actions will fail for a certain period of time. Most users will never hit these limits, as they are well above the activity level of a typical Calendar user.
When a user makes changes to non-primary calendars (such as secondary calendars or a manager’s calendar), all actions are counted under the user's own account and not the limits of the processed calendars. Google Apps administrators who use Google Calendar heavily or anyone using a third-party tool or API to manipulate events therefore have an increased risk of hitting Calendar limits.
If users hit the Google Calendar limits, they should be able to edit their calendars normally again within several hours. Users will still be able to view events in Calendar during that period.
If a user sees one of these messages it’s probably due to one of the following reasons:
Creating too many events
If a user has created more than 10,000 events in his or her calendar within a short period of time, that user might lose calendar edit access.
Creating too many calendars
If a user creates more than 25 new calendars within a short period of time, that user's calendar might go into read-only mode.
Sending too many invitations or emails to external guests
In order to prevent spamming, Google Calendar limits the number of invitations a user can send to external guests. This limit varies depending on the action, and is usually between 100-300 guests.
Google Apps users can send invitations to any number of guests from their primary domain, or from secondary domains associated with their primary domain.
Sharing calendars with too many users
If a user shares one or more calendars with many other users within a short period of time, Google Calendar might switch into read-only mode for that user. It’s almost impossible to reach this limit by updating sharing settings manually, but it can happen with some API-based tools or third-party apps.