Estimating Server Capacity

Hi Everybody,
I am trying to forecast the server capacity costs as my app scales. Does anybody have any helpful experience with this. In particular, what kind of usage were you seeing in your app/sub apps before you had to add additional server capacity units. Thanks in advance!

With a very data intensive database with 15, 000 data workflows daily, It only hits capacity of the personal plan ($16) for about 30 minites a day. This is roughly 300-400 daily users but very data intensive.