If the SwitchOnRebuild feature is enabled when using the Azure Search provider then each index rebuild operation creates additional schema synchronization tasks.
Over time the number of requests to Azure Search service increases to the point when new requests cannot be processed:
Exception: Sitecore.ContentSearch.Azure.Http.Exceptions.AzureSearchServiceRESTCallException
Message: {"error":{"code":"","message":"You are sending too many requests. Please try again later."}}
To fix the issue, download and apply the hotfix for the affected Sitecore version.
Be aware that the hotfixes were built for specific Sitecore versions, and must not be installed on other Sitecore versions or in combination with other hotfixes that modify the Sitecore.ContentSearch.Azure assembly. In case any other hotfixes have already been installed on certain Sitecore instance, send a request for compatibility check to Sitecore Support.
Note that ZIP file contents need to be extracted to locate installation instructions and related files inside it.
The hotfixes must be installed on CM instance and then synced with other instances using the regular development practices.