set check-interval

SET CHECK-INTERVAL seconds  -  define how often to check for new mail
Usually:           300

The CHECK-INTERVAL variable defines in seconds how often MM checks whether
  new mail arrived in /usr/spool/mail at an idle terminal.  If CONTINUOUS-
  CHECK is set to its usual value NO, the check is done only at Top-Level 
  (the MM> prompt).  A check is always done after each command is finished,
  without reference to the CHECK-INTERVAL variable.

The usual setting is 300 seconds (5 minutes).  The setting is a compromise 
  between wasting CPU time and getting mail notification promptly.  Values 
  under 60 (1 minute) are not recommended.  The value zero means no check 
  is to be done while MM is idle.

You can also use the command CHECK at any time to check for new mail.

For more help, type "help" and one of the following:
(at any prompt:)  save-init   set   show   check   set continuous-check