auto-revert-interval is a variable defined in `autorevert.el'.
Its value is

Time, in seconds, between Auto-Revert Mode file checks.
The value may be an integer or floating point number.

If a timer is already active, there are two ways to make sure
that the new value will take effect immediately. You can set
this variable through Custom or you can call the command
`auto-revert-set-timer' after setting the variable. Otherwise,
the new value will take effect the first time Auto Revert Mode
calls `auto-revert-set-timer' for internal reasons or in your
next editing session.

You can customize this variable.