The 'sleep' value is set to 1200000 milliseconds, which equals 20 minutes, but can be changed easily. The message title and message body can easily be changed as well by changing the values of the initial variables.
intSleepTime = 1200000
strMsgTitle = "Rest Time!"
strMsgBody = "Get up and stretch and rest your eyes!"
While intMessage < 2
intMessage = MsgBox ("Rest time!" &VbCrLf&VbCrLf& strMsgBody &VbCrLf&VbCrLf& "Click Ok to continue or Cancel to exit.", 65, strMsgTitle)