My theory.
Apple releases the new Apple Watch - April 24th, 2015. Worldwide rollout takes until the end of the summer, to early fall.
New editions released by the Fall for the Holiday Season to promote the watch. Probably during the iPhone 6S Fall event. No internal or design updates just new materials for the casings and bands. A perfect gift for the holidays.
Apple will showcase new updated watch OS at WWDC 2016 along with iOS 10 and the next OSX.
New Apple Watch launches alongside iPhone in September 2016 iPhone event. 17 months after "first" generation model launch and 2 years after announcement.
I don't expect a redesign, I expect more sensors, "S2" processor and better battery life. Perhaps an upgrade option for the first Apple Watch offered as well. atleast for the battery.
The cycle will continue to be yearly at the iPhone event. Redesign perhaps every two or three years, maybe in 2017. I do not expect the watch to change much. I think apple will stick with square design, digital crown and friends button/retina display. It will get a little thinner, probably about as thin as the crown itself. Focus will be on battery life and sensors!.
The reasoning behind my theory for not seeing a second generation Apple Watch until the end of next year 2016.
1)The Apple Watch is already pretty powerful - There was an article that the "S1" processor is on par with the fifth generation iPod Touch or the A5 chip. For a smartwatch, that is pretty slick. Most tasks will be performed by the iPhone. No need to watch 1080p video on the Watch

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2)Requires iPhone - iPhone performs many of the tasks of the watch. Therefore upgrading to a new iPhone can therefore make the watch more powerful.
3)Pretty Thin for a Smartwatch - 10.5mm seems bulky for someone who doesn't wear a watch or who hasn't seen the plethora of android wear smartwatches. My Stainless Steel Armitron Analog watch is about 10.5mm. So a 10.5mm smartwatch to me is pretty svelte.
4)Battery is Upgradable. Hopefully Apple is smart and when the second generation Apple Watch is introduced they allow first generation users to upgrade to a better battery, no just a replacement. That would be awesome! So 18 hours could become 40 hours of mixed use

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So don't expect your new Apple Watch to be absolutely obsolete come Spring next year, I think the watch will be an every 2 or 3 year upgrade for most people, maybe more. Remember: Replaceable battery
