an EoS date has not been announced

It's hard to predict the EoS for a specific hardware, but for those regular BigIP platforms, I'd expect at least 5-6 years between beginning of sale and end of sale. Of course, there's no guarantee for that. Since the C113 platform began shipping in Oct 2012, I'd not expect EoS before end of 2017, but again, that's just a guess, no one here can give a reliable prediction.

Keep in mind though that the EoL policy "normally" provides extended support for four years after EoS announcement: For platforms, unless specifically declared and announced by F5, four years after the Platform EoSD date is the default platform End of Technical Support (EoTS) date and also the End of RMA (EoRMA) date.