You don't necessarily "learn Ionic", you need to know Angular/HTML/CSS if you want to start using the Ionic framework.
While there is a lot to do, ionic basically gives you a bunch of custom angular directives to use in order to make your program feel like a native app. If you are comfortable with angular, you should be able to pick up Ionic in a night.
But learning Angular/Javascript is where there is a steeper learning curve. I've been writing Angular apps for almost 2+ years now, and I'm still learning stuff every day. Its all about how much time you put in.
I bet you could publish an app within 4 months if you started now. Spend a solid couple months learning Javascript and Angular, then pick up ionic.