I planted lots of trees last winter and have been watering them once or twice a week but with a good soaking. They have weed mats around them which I think helps retain the water a bit better as well. All are still OK but we could really, really do with some rain here (has been threatening for a couple of weeks but with pathetically little falling). If you've planted this summer, you might need to be watering more - at least mine had a bit of chance to get their feet down in the spring.
H