You can make modifications to the number of outgoing emails an account may send in WHM>>Server Configuration>>Tweak Settings -> Mail
I know a lot of folks use about 100/hr but there's no standard, it really depends on what works for you and doesn't inhibit your users' legitimate tasks.
There is no standard limit, set it a low limit, say, 50 per hour and you can raise it for each account according to the needs.
