All of the products you list are hardware, which doesn't count. The only exception is Google's social stuff; Google had shipped many social products to get where they are today including Gmail, GTalk, Orkut and Buzz. They didn't sit in their offices and build Google Plus for 10 years, they iterated again and again, and that's what Mayer is asking for.
GMail took 3 years from prototype to launch. Google Search was also 3 years from prototype to incorporation. G+ was shorter, but was built on previously-existing infrastructure that's been around for a few years.
Now, it's better to spend those 3 years iterating than planning - rumor has it that the GMail prototype was built in a day. But it's worth being realistic about how long it takes to build a world-changing product - it's a long road. (In fairness to Marissa, I'm sure she knows this, and I suspect she's just trying to get the release cycle down so that Yahoo can make forward progress.)
There's a big difference between major projects and adding features too.
Maybe this is not the right time for Yahoo to do 3 year products simply because they suck at it (I spent more than 3 years at Yahoo, and I spent most of that time trying to shepherd projects through to the point my engineering team would actually get a go-ahead; we were a service function without direct control of a product, otherwise we should've just gone for it rather than wait...), and practising on 6 month projects could still bring a ton of useful improvements or smaller services. One thing to keep in mind with Yahoo is that it has a vast array of sites and services just sitting there - I bet there's a ton of poorly monetized services that could get drastic upgrades in well below 6 months if there's just sufficient fire behind someones asses...
Then they can start trying to add the standalone world-changing stuff again later, if they're successful.
Games/Music/Hardware/Software always iterate to completion - the web isn't special in terms of this. The key component is and will remain time to search through a product space to where you can actually add value.
What pbiggar is saying is that hardware doesn't make sense to measure on the same timescale as software, not that it doesn't involve iteration. Think about the "time to ship" of sites like Facebook, Twitter, Youtube, etc and you start to see numbers closer to or within the 6 month time range.
FB, Twitter, and YT were incredibly small teams of less than a handful who knew exactly what they wanted to build. Expectations were minimal or non-existent when they launched. They didn't have a corporate board, a CEO, or Wall Street looking over their shoulders for monetization.
Looking from the outside, they seem to have a dysfunctional culture around shipping software with all the additional layers of management. Hopefully this changes and they have a chance producing meaningful products but I'm not hopeful.
