As much as you can afford. With pianos, you typically get what you pay for. In other words, the more expensive the instrument is, the more likely it will be of a better quality. Learning on a bad instrument can be like learning to drive on a golf cart. Of course, it will drive...but it's just not the same as learning in a car. Moreover, the experience can severely impact your enjoyment and ability to learn proper driving skills on it. Despite the high upfront costs, a good, consistently maintained piano will last you for decades. So in the long run, it will pay off.