F5 Analytics iApp

Does anyone have any sizing recommendations for using splunk with F5? I've got the free version of splunk and have overwhelmed it with just my non-prod LTMs (in an active/standby pair) - I've got four more pair and have no idea what I would be looking at as far as storage to request or size of splunk license I'd need. I've contacted splunk for a larger license for a POC but I don't know if the busier LTMs will send more data vs the less-busy LTMs (I presume so) or how much.

I'd like to know what others have experienced here. This looks like an awesome tool but I won't get a blank check for licenses & storage. I have five pair of LTMs (so far) and would like to have at least a month's worth of historical data for trending. At least that's what I think - what's your experience?