My planting proposals specify that drip irrigation, meeting my specifications and able to deliver the volume of water at the interval I specify, will be in place and operational at the time the trees are planted.
With that in place, I guarantee them for 2 years, no problema.
My contract excludes insects, diseases, acts of war, fire, storms and temperature extremes, since I can't do anything about that.
Some of the restoration work, for which I've planted, has county requirements that the trees be guaranteed for 2 years.
Dr. Ed Gilman has a handy-dandy establishment chart on one of his site-evaluation power points, which cross-references trunk caliper and hardiness zones to give you a figure of how much water, how frequently, and for how many months to achieve healthy establishment. I just specify the correct amount for the trees, and haven't had a problem