I believe first 1 -1.5 yrs though I would refer to Treesny.org for more info. They run Citizen Pruner courses for NYC trees and the main guy there, Sam Bishop, was a great resource for me. From their website: What can I do to help take care of a newly planted street tree?

WATCH SAM’S TREE TIPS VIDEO

For newly planted trees that most important thing you can do is to water them. Form a ring of soil about 2 feet wide, with the tree in the middle, and fill it with water. That keeps the water near the tree’s roots. Water the tree with 15-20 Gallons once a week. It is much better to give the tree more water less often, then to give it a little bit every day.

You can also cultivate the soil. Cultivating breaks up soil compaction, so water and air can get to the tree’s roots. Soil becomes compacted when people walk on the soil, or when heavy objects like garbage bags are placed on the soil. Dig 2-3 inches down into the soil, and work around any large roots you find.

You can also add wood chip mulch to the tree bed. Mulch is chipped or shredded trees and branches. Mulch protects and improve the soil, and reduces water loss, so there is more water for the tree. Use 2-3 inches of mulch on the soil, but don’t let it touch the trunk of the tree. If it touches the trunk for too long, it can cause disease.

From: https://treesny.org/community-resources/treesny.org