1 week to get u started ( the max) and depends on the projectes u do.. u learn more.. and your commitment about the quality of code.. ( atleast 6 months to call yourself a pro !!!! )
PHP is one of the easier languages to pick up and learn. Do you have any prior programming experience ? As in do you understand what a while loop is or If statement? If you do then you will pick up PHP quickly, if not then it may take a little time. I can highly recommend this book: http://dev.mysql.com/tech-resources/articles/ddws/index.html as great place to start as it guides you through creating a full website from beginning to end. Just know however the first few chapters are about how to install and configure PHP and MySQL, which you dont really need to know as long as your web host already has it on (most do). If you dont want to do it on a host, then you can run MySQL and PHP on your computer by installing WAMP Server, which does all the config for you! http://www.wampserver.com/en/
Go to your nearest bookshop, buy a good (basic) PHP Guide. That's how I learn. I can write a fully-functioning website after 1 month and a chatbox service after few months. :: ads2help
this is a great place to learn php http://www.w3schools.com/PHP/DEfaULT.asP i am learning it there myself
PHP book tutorial: http://www.hudzilla.org/phpbook/ As for "how long" it'll take, it depends on what you're gonna be using it for. If it's just for some basic scripts, then you may get away with learning some basics and then downloading one of the many php frameworks to get you on your way. If you need to do some complex stuff, then it could take a good while longer because you're probably gonna want to learn how to use a database such as MySQL. If this latter scenario is more likely then I actually recommend you learn how to use MySQL first before diving into PHP.
Also forgot to say, get yourself and editor that makes life easier. Everyone has their own favourite for different reasons, my person favourite is the Notepad++ just because I use it for everything.
It will take you a couple of years to get good at it but in a couple of weeks you can be somewhat proficient at it. Enough to edit open source while adding a couple of your own touches. But this depends on the fire within to want to learn a new language.