Hi there!
You'll learn a lot about music by studying for the grade 5 theory exam, even if you don't actually take the exam. I know a lot of people think that the theory side of things is a bit dull or not really connected with real music making, but I've never met any musician who regretted learning something about theory. The good thing about the ABRSM exams is that they are structured in a logical and accessible way - and if you do get to the stage where you are ready to sit grade 5, you'll feel a great sense of achievement.
As for how long it takes, it really depends on what you know already and whether you are a fast learner. As someone mentioned, having a good brain for things like fractions can be a help, but above all, having an interest in learning is the most important thing. If you want to learn something, you are much more likely to succeed than if your heart is not really in it!
I say, go for it!