That is something that will vary wildly from person to person. I don't think any programming is a matter of "practicing", but instead it's a matter of learning the syntax/patterns and understanding how to apply it. If you understand one programming language, or at least a workflow that you would like to automate, then it's a matter if translating to the programming language you need to use, making sure (syntax) spacing is correct, brackets are balanced, etc., and following all the other rules for the language.
How long or how often you need to use Python to become proficient with it will depend on how you learn. Are you wanting to sit down with a blank sheet of paper, with a problem, and no other resources (that is, no Python books, no help system or online forums, etc) and want to write a perfect program from scratch? If so, then you'll want to memorize the language. But most programmers do not work that way. Most have books and/or better yet, the Internet for resources....and one of the best ways to learn any language is to read thru code that others have written and see if you understand. Also, most if not all of us at some time or another have grabbed others' code and modified to our needs. Even the ArcGIS help pages have code to start with.
So how and to what level you learn is a personal goal, not a one size fits all. With that said, there are many discussion (on Geonet and other forums) about Python books and how to learn, For ArcGIS you will want to study arcpy, not just straight Python.
LLook thru some of these posts:
Introduction to Writing Geoprocessing Scripts with Python 10.2 Class Resources
Any Good Python Books
Using Python List Comprehensions
Python Map Automation – Introduction to arcpy.mapping
Or do your own search. Also, following Python is also a goid start.
Have fun.....that's always the best part...
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