If you're just talking about the functions, syntax, session handling etc, if you're already an experienced programmer it might just take half a week - if you keep the online function reference handy...
That's assuming you've been programming web apps before, or at least have a good understanding of http and databases etc
If you're talking about slightly larger projects, you might have a bit of trouble to adjusting to the different approach to OOP patterns like MVC - it's quite different to event driven programming.
It's crucial to look at how the most well organised applications are structured. Try delving into some open source code like that of gallery2, wordpress or similar - something like the applications you're hoping to go into designing / programming.
There's a difference between writing masterful programs and mastering PHP - actually mastering PHP is totally up to how well you learn surely?
