As far as I know, there's no best practise for uptime, it should be working just fine without any reboots.
To investigate what may be the cause though, have you tried running a qkview and analyzing is using iHealth? It should highlight any major problems in the system.
If unsure on how to do this, in the GUI, go to System - Support and then create a Snapshot. Once finished, go to ihealth.f5.com and upload the file there.
