The default setting for Thunderbird to check for new mail is 10 minutes. Some people prefer to get it more often than that (I know I do). To change this setting, go to Tools > Account Settings. This will open the Account Settings options. From here, click on the “Server Settings” option listed under the email account for which you want to change the time. The second section, labeled Server Settings, has the setting for “Check for new messages every _ minutes”. Change that number to whatever you want and presto!
How to change how often TB checks for new mail