Use an already working React or Angular app with Meteor

Hi, I’m considering Meteor as a backend and want to know how difficult it would be to use an already working Angular or React app with Meteor now that it supports them officialy.

What modifications will I need to do and how much time approximately will it take (assuming the Meteor backend is ready).
I just want to have a brief explanation about converting an Angular or React app for Meteor.

Thanks.