Thanks for asking that question about how long and how often to irrigate. Looking forward to the answers. Last summer the county cut back the watering due to the drought. We are mandated to only water on certain days. We have 3 trees with one gallon emmitters, and water for 45 minutes about every other day. One tree is 4 years old. The others are new. Fingers crossed the county remembers they already cut us back.

better than drip are micro sprinkler.. not sure how big your trees are but you could dig a few holes and put some 4 inch perforated drain pipes in that are at least 2 feet deep. this will allow you to deep water below the surface. unless you leave drip on all day it will only water one spot and just a few inches deep before it dries again
the drain pipe is best when planting a new tree but you can add it later roots will grow back

I have grown avo trees for 40 years....mostly via drip systems. My recommendations: For very small/young trees the drippers are fine. It is important to target in the small rootzone. After trees are in the ground for a year or two....go to the micro sprinklers that M is talking about. They cover a larger area...and this reduces the number needed. Young trees need more frequent watering than mature trees. However, do know that avocados love water. Standing water/flooded ground is not good....and leads to root rot. Giving timetables for water is impossible because of the many variables. Mulching areas around trees is very helpful. Hope this helps...

I have a soaker hose set up on all my fruit trees. Costs a little more, but worth it.