Under the Tips and Tricks sticky at the top of this forum there is a link to a full Eris manual, you should probably go and get that document. It will help answer many of your questions.
For setting frequency (per the manual);
Editing email account settings
1. Press HOME, then tap Mail.
2. Switch to the email account you want to edit.
3. Press MENU and then tap More > Settings.
Account settings Change the email account settings such as the name,
email address, password, and description.
General settings Set the font size when reading emails, enable,
disable or set the signature, set the account as the
default email account, and set where to save email
attachments.
Send & receive Set the maximum email file size limit when receiving
and frequency to check for new emails and how
many to check for. You can also set to have emails
that you send bcc’d to yourself.
Notification
settings
Set email notification settings.
Delete account Tap to delete the email account.
4. Press BACK to save your changes.
