There’s no single answer to this question as everybody’s results will be different. How much you make depends on various factors such as the amount of time you spend on it, your starting bank roll, whether you have any existing bookmaker accounts and whether you want to utilise the high street bookies to name a few.
What I can tell you is that making £500 per month is easy. I’ve seen people claim on matched betting forums and Facebook pages that they make £20k+ a year. That would be difficult unless you treat it like a full time job, but £6k a year is easy, £10k a year is certainly achievable. And remember, it’s tax-free.