Observer - Total Time

I'd love to have the ability to use the total time (billable/nonbillable/or both) as a condition for Observer.  We have a custom field for "Max Billable Time" to indicate what we've quoted a client, and I'd like to be able to create a notification when we've gone over what we've quoted or are getting close.  Based on the current conditions, I'm not seeing a way to do this.