I don't think there are any ways to "estimate" development specifically for ExtJS... it would be the same as estimating development time using any other toolset. The estimation is based on the number of developers you have on your team and their skills, in addition to the complexity of the project at hand.
For coding standards or best practices, check the development guides in the ExtJS API docs and the Sencha blog.
The more you use ExtJS the more familiar you will become with all of the components. Once you get to that point you'll have a better idea how long a project will take based on the components that project will need. Plus, I would recommend learning the ExtJS MVC approach since that will make your project easier to maintain for future enhancements. Some may argue that it is more work up front in setting up a project to use the ExtJS MVC approach but from my experience, this was only true during the initial learning phase of how to do the ExtJS MVC stuff. Once you learn it, it is actually faster IMO to develop projects this way.