Normally, you can select and delete only a screen’s worth of mail at a time. If you want to delete thousand emails in Gmail, please follow these steps:
1. Login Gmail.
2. In Search, type the the date you want to search, for example, before:2010/01/01, then click search icon.
3. In the Select session, click All.
4. You will see Select all conversations that match this search. Click on it.
5. Click Delete icon to delete all searched emails.