PerProcessUserTimeLimit specifies the amount of user-mode time is granted to the process.
"The system periodically checks to determine whether each process associated with the job has accumulated more user-mode time than the set limit. If it has, the process is terminated." (MSDN)
Consequently it depends on your application, particulary on how effient it is burning user-mode time. Ending a process with PerProcessUserTimeLimit = 0.5
after 0.5 sec. means that it has used ~100% cpu (user-mode) during that time.
... if Job Objects are executed periodically or in real time? Periodically, as stated above.