There are three ways people typically use "temporary" mail with Gmail, and each has its own "full" trigger:
Third-party sites (like Temp-Mail.io or Temp-Mail.org ) that generate a fake Gmail-like address for one-time use. gmail temp mail full
Refresh the page to generate a new address or manually clear the temporary inbox. For Real Gmail "Burner" Accounts: There are three ways people typically use "temporary"
Google provides shared across Gmail, Drive, and Photos. If you use this account for "temp" purposes, it can fill up surprisingly fast with: gmail temp mail full