I wouldn't take the eris update timeline as a good measure of how long it'll take. Remember they were jumping from 1.5 to 2.1, which is a major leap and alot of testing needed to be done.
In fact there were 3 leaks of the 2.1 update before the last one got pushed to the phones. Which means they probably intended the first one to be released but noticed bugs and sent it back to the devlopers at htc to iron out. Once htc signed off on the second release, verizon got it and tested it some more. They found some bugs were still present while others were cleared up. So they once again sent it back to HTC and then HTC took their time and really give got things ironed out. They then sent it back to verizon for the 3rd time and verizon did their testing. Verizon was happy with the results and then opted to push it to their subscribers.
Its important for verizon to maintain a high level of customer satisfaction at this point in the game. Any slip up can give their competition the needed ammo to boost their own marketshare. Verizon did the right thing and now its being rolled out via OTA and should finish up sometime next week or so.
All the while they have been working on the incredible to get it ready for launch and were buttoning up their negotiations with google over the nexus one (which they opted to not go forward with). So verizons had alot on their plate the past few months. If you consider the fact that this was done right after the holiday season, they haven't had a chance to breath over there. Oh not to mention they rolled out the OTA update for the moto droid while this was all going on.
The fact that we got our first OTA update for the incredible within a week of the phone being released should be an indication that HTC is taking this phone very seriously and verizon wants to make sure everything is just right for us incredible users/subscribers. Imo the 2.2 update will come much quicker than the eris update as its only an incremental update, not a quantum leap like going from 1.5 to 2.1.
Google also wants to cut down on os fragmentation, they don't want users strung out over 4 different version of their os. So imo they will do what it takes to get everyone over to 2.2 asap. The longer it takes the more heat the developers will put on google. Right now i couldn't imagine developing for the android platform, it must be a nightmare. When they can get everyone consolidated to one version of the os it will clear up alot of problems. I mean what is the advantage of developing for 2.1 right now? When so many users are still on 2.0, 1.6 and even 1.5. If you add 2.2 on top of that, it will affect the market dramatically in a negative way. Imo google is really going to push 2.2 hard once their work is done, so it'll be a matter of the other companies doing what they need to do to get it done.
Also its my opinion that this is one of the reasons the eris is getting 2.1 now, so the upgrade to 2.2 won't be such a huge leap. I could see there being problems going from 1.5 directly to 2.2, expecially with such a drastic change in the core processing procedures. So it only stands to reason that pushing all phones to 2.1 asap is their top priority until 2.2 is ready to go live.