While I don't think there are any hard rules, I think this is (about) in line with what I am expecting ... Basically, Nintendo will announce and release their next system in 2012.
Probably the next 6 months will be approaching major third party publishers with details about the hardware and strategy of their next system and trying to encourage support for their system. While few details of this system emerge, speculation will run rampant and some analysts (Pachter) will predict that Nintendo will announce the system at E3; E3 2011 will come and go, and be primarily focused on 3DS games that are for Nintendo's holiday releases.
Around March 2012 Nintendo will be forced to announce the existance of their next system because they will need to include in in sales projections for their next fiscal year. Details will be limited at this point in time. E3 2012 will come along and Nintendo will showcase their next system and it will be released before Christmas 2012; possibly much before christmas 2012.
