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You really can't know.  Paolo's suggestion is great, but even Cisco doesn't really know.  Reason being so much depends on what's happening in the market, how large is the installed base, new purchasers, how easy it might be to expand hardware features, and what the competitors are doing.

Anything brand new is likely to be around for at least 3 years.  Appliance type platforms (e.g. 3750) tend to have shorter life times, chassis type platforms (e.g. 4500/6500) longer life times.  The latter probably good for at least 5 years. ISR come and go, but 7200s are still available.

Because of the market, some devices die much earlier than I think was planned.  Some live on much longer.  For example, the original 3750 series included a model with a 10 gig port.  The series and its successor series have been around for a while now, but that particular model disappeared in a flash.  Or, there was a 6502 10 gig line card, that came and went so quickly, most probably didn't notice.

On the other extreme, I thought the original 4500 series was going to die like the 5500 series, but the E variants extended their life.

So, again, Paolo's suggestion is great, but whatever Cisco's roadmap is, the market might create unexpected "detours".