So it looks lie the current behavior is:Memory evaluated (above threshold) > 30 minutes wait > memory evaluated again (still above threshold). The easiest way, assuming the data is collected every 5 minutes, you can change the settings to fire alert after 12 consecutive evaluations. If you want to have something that alerts if you are over the threshold most of the time, you can use the Fire actions if  X out of  Y evaluations are true. If that doesn't work, there is probably a way to change the rule condition  itself to check for average utilization for 60 minutes but let's try the easy solutions first.   Golan    
Hello,  how can I reduce how frequently these emails go out? I get hundreds a day.  [Fatal] Memory utilization on virtual machine [machinename] has reached 96 % of the memory ESX has granted to it.  The following URL can be used to obtain alarm details.   I've adjusted the behavior to "Fire action if 2 consecutive evaluations are true" and "wait at least 30 minutes after first evaluation". 
Memory Utilization Emails