Its a bit of a 'how long is a piece of string' question. And the answer is 'depends'
If you paint a large area fairly regularly you'll use it pretty quickly. If you paint fine art using drops at a time regularly, it can last a year. If you want full opacity and more layers, you'll use more. If you predominately use particular colours more than others, you'll use it quicker. Etc. If you buy a lot of bottles of different colours, they could last much longer than if you just used primary colours plus black and white to mix all your colours. If you need to colour match precisely, you could use more paint or waste more before you get the exact blend of colour. There's a lot of factors that may be different for everyone.
But as JackEb said, with reduction a bottle can go 3,4,5....times as far. It goes a long way.