There is no schedule, and it's decidely retarded down as Android has grown. I'd stick the big UI modifies thus far, as Gingerbread "12-2010", Ice Cream Sandwich "10-2011", and Lollipop "11-2014".
My supposition is that the flow basic UI design will last at least a some years, but that is only an educated guess.
I guess a lot will depend on what you mean by a large update.
Android is continually in development and minor updates are pretty regular. A lot of these won't impact too much on the way your device works, a lot are more security fixes and adjustments to efficiency of how the platform runs.
Lollipop 5.1 is the latest big update if you want to call it that and I think this update is currently being rolled out across most platforms as we speak. Now this will impact on the way you use your device and you will notice a lot of changes.
I think some people are a bit sceptical about installing the updates as everybody customizes their devices for how they want to use them but if an update is available I'd certainly recommend downloading it. While it might take a bit of getting used to, on the whole the android updates do improve the features and it's important to make sure whatever platform your using is up to date.
I think it is more often than other devices at the moment. It would actually depend on the market and fixing certain bugs and the upgrade of new devices on the market. The higher specifications that get manufactured, the better android system it will need, as well as the development of applications and the increase production and need for RAM. They all play a major role in it. I believe that Android would change more frequently than Windows and other certain OS for the smartphone because the more people use them, the more apps will be developed and upgraded, as well as phone manufactured with new system requirements, therefore, more upgrade of Android OS and fixing of bugs that will be suitable for the new devices and the new environment.