For sake of example, I will assume that the max not to exceed is over all time billing data for the same project. The same method shown here works for a limit for one employee's work on a given project as well--you'd just set up a different relationship.
Set up a self join linking Time Billing Data to another occurrence of the same base table by _fkProject
Time Billing Data::_fkProject = Time Billing Data|SameProject::_fkProject
a calculation field define to evaluate from the context of Time Billing Data might use this expression:
If ( Projects::MaxTime - Sum ( Time Billing Data|SameProject::Hours ) < 10 ; "Warning: Max Time Limit close or exceeded" )
To show a warning when the total time is within 10 hours of Projects. A Case function might be used to display multiple messages.