The development community will have this out long before 2 years. My OG Evo had ICS 2-3 months after the source code was released. The average phone user wont get Jellybean for a short period, but they are usually the ones who know nothing about these updates. A lot of android users I know still dont know what ICS is and are still on 2.2. let alone 2.3
I'm sure this'll be an update similar to the 2.2--->2.3 update. It'll add some new features, tweaks to make it smoother, update to apps etc. There wont be any drastic changes compared to 2.3-->4.0.
Not to mention that since this is an incremental update and not a complete overhaul like ICS was, it will be easier for manufacturers to implement the update. ICS has taken longer than others because it was a huge overhaul of Android OS. It even took a while for the open source community to make ROMs and they are usually very fast. Jelly Bean will be a lot easier to update.
As for the two year remark made, ICS has not even been out a year and most of the major phones have the update. The rest of the compatible phones will have the update before a year of the release of ICS. ICS was released in October of last year and the first phone with ICS (Galaxy Nexus) was released in November. It has been eight months, no where near two years. The older phones will not get ICS period. That percentage that keeps being mentioned is stupid. ICS will not run on low end phones with less than 512MB of RAM. Those phones will never get ICS. All compatible phones will have ICS before a year's time of the release.