Nimbostratus
The optimal frequency of calls is very dependent on what the calls are. If all you are doing is making status requests, then the overhead is very low and every 15 seconds should not cause any burdon on the system for your 5 servers (or additional three). If, on the other hand, you are making requests for large amounts of statistics, then you might want to space out the requests a bit longer.
The BIG-IP serializes iControl requests into a simple FIFO queue so that only one is executed at a time. So, the more servers you have making iControl calls, the only side-effect will be that they have to wait a fraction of a second longer than they normally would if acting alone.
Hope this helps...
-Joe
