In all honesty, you might never "master" it, so we really can't answer that
question.
This question came up not too long ago in the VB newsgroup.
As a beginner, you don't know what it is that you don't know and there is
more than just knowing VB .NET to be able to use it in a meaningful way.
VB .NET is a programming language designed to allow a programmer to write
code that interacts with Microsoft's .NET Framework and its internal Common
Language Runtime (CLR), which is what really does all the operations you
need done.
It turns out that this CLR doesn't speak VB .NET, it speaks Microsoft's
Intermediate Language (MSIL or just IL). But before you go and run out to
learn IL, don't worry because the Visual Basic .NET compiler's job is to
turn your VB .NET code into IL for the CLR to be able to run it.
Why is this important to know? Because since it is the CLR that ultimately
does the work, you need to know a bit about how the CLR works and what it
can do for you. VB .NET is the language you'll use to send instructions to
the CLR.
You can learn the fundamentals of the VB .NET language in just a few days
(decision making code, loops, operators, data types, reserved words etc.),
but it can take literally years to know how to use it correctly.
-Scott