The Google Calendar API has a courtesy limit of 1,000,000 queries per day.
To view or change usage limits for your project, or to request an increase to your quota, Open the API Library in the Developers Console.
If you have enabled billing for your project, you can see the quota percentage for each API. You can view and change quotas from the API section's API Library and Enabled APIs links. Selecting an API from the API Library list or from the Enabled APIs page takes you to a page with overview, usage, and quota links for that API. Choosing Quotas lets you view and change quota-related settings. Choosing Usage provides you with usage statistics as well as response code and method information.