Activations last indefinitely - well, unless Adobe really decides to not support older versions. People happily activate CS/CS2/CS3 and so on even today. Anyway, in time this matter will settle itself because at some point your Win 7 won't run on any computer any more... Whether or not CS5 will work on Win 8 is another matter. Depends too much on the specifics. In your case most likely graphics card issues will be a problem in Premiere and After Effects and Mercury playback and other acceleration stuff not be available. Many otehr people struggle with more general issues like funky screen behavior or apps not printing with Win 8's new printer driver model... In short: Results are mixed and you won't know until you try.
Thanks for explanation about activation, that was helpful. But why say "Win7 won't work on any computer"? I assume as long as I don't change my hardware, both Win7 and CS5 should keep working forever! The question is: will it still work if I upgrade to Win8? Any experience? Maybe the golden rule applies here: "If everything works OK, don't change it"?