It's hard to say how much water to apply, but I would start with a couple liters every 3 or 4 days. A lot depends largely on the kind of soil you have. Lighter, loose sandy soils don't hold much water, so you'll need to water more often if regular rain doesn't fall. If you have heavier or dense clay like soil, they hold a lot of water and you won't need to do it as often. When they fully leaf out and put on vegetative growth their water needs will be greater than they are right now with just nubbins of emerging growth. Just keep on eye on the soil at the base of the tree. If it looks and feels damp, skip the water. If it seems dry, add some. If the leaves start to wilt, things have gone too far and gotten way too dry, and after watering, the tree will spend
energy growing new root hairs that died from the lack of water in the soil, further stressing the tree. Basically, just keep an eye on it this first year and don't let it dry out and don't let it get waterlogged.