For trees with larger root balls, you want to provide a deep watering. To do this, the hose needs to be left at the base of the tree while the water flowing at a steady (but not overwhelming) flow. Allow for 15-30 minutes for the water to soak down in the root ball. If you do this properly and allow it to be absorbed, you only need to water in up to 2 times a week (assuming the rootball was big, and can absorb alot of water). As with all plants there is not a specific watering schedule because there are many variables that come into play (rain, soil type, root ball size). You can always dig around at the root ball and see if it is dry too.
The first few weeks after being planted are important, as they need water. For the first 2-3 weeks, put the hose/nozzle at the base of the plant and let it soak in deep for a bit. If too much water is running off, then reduce the flow rate at the faucet knob. Again, the goal here is a deep watering, not a quick drink (Quick drinks are fine in the weeks after, but not right now.) Once you think they are fully saturated you will be good for 2-3days (especially if it rains steadily in-between your watering schedule), so dont do this process everyday or you will be overwatering them. On that same note, if its super hot out, the water will evaporate faster though. Lastly, after a few weeks, you can change to just giving them a quick drink here and there as needed based on the weather. It'll take weeks/months for roots to be established, so watering them occasionally is no harm at all.
Notes: Signs of over-watering can be yellow leaves appearing or heavy drooping. Also, if the plant starts to look like they are dying but you know you are watering them enough, then you may be watering them too much, too frequently.
Inversely, signs of dehydrated plants are shriveled, wrinkled, curled up leaves. Look at them now while they are healthy so you know what normal leaves look like.
Regarding rain: unless it was a steady rainfall for 4-5hours, then the plants need watered (maybe not as much but still needs watering.) A quick heavy rainfall will probably not sink into the root ball far enough, if at all because the mulch and plant leaves will also stop the water from reaching the root ball too, so keep that in mind.