how much water does a potted olive tree need

Olives are used to hot, dry climates, so in the ground, they do fairly well, once established, with not much water. However, yours is in a pot, which means you need to water it whenever it needs it. When it's hot, give it a good soak once or twice a week, when it's cool, check it once a week - even when it rains, it may need watering. I assume you've got holes in the pot for drainage so that excess water will run out. If the plant cannot drain down, that would make the leaves yellow. If you're not sure how wet it is or isn't, turn it out of the pot and check the rootball. And when you say spotted, what kind of spots? Black, brown, pitted, raised, what?

As Bamboo says , the olive is from the Mediterranean region and prefers a hot, dry climate. We lived in Spain for a few years and had our own Olives, and this yellowing and spotting I'm sure is a fungal disease. If you are watering it too regularly the air around the Olive will be too damp, attracting fungal spores. It will need good air circulation to keep it healthy. I certainly wouldn't soak it daily or feed it weekly, you will end up with a very weak specimen. They are very tough trees....used to extreme heat and hot, dry winds, ours even survived winter snow falls.