In India , it is growing even where 400mm of annual precipitation. So requirement of water to neem is very minimum

Nazir, a web search gives this info:

HOW MUCH WATER DOES IT NEED ?Neem tree needs little water and plenty of sunlight. The tree grows naturally in areas where the rainfall is in the range of 450 to 1200 mm. However, it has been introduced successfully even in areas where the rainfall is as low as 200 – 250 mm. It cannot withstand water-logged areas and poorly drained soils. Neem makes land more ferti

Water:

An ample supply of water is good when growing neem trees, but the tree can make do with very little.

The areas where neem trees grow naturally receive between 450 and 1200 mm of rain per year, but neem has been planted in drier regions as well. There are neem trees growing in areas that receive as little as 200 mm annual rain.

The one single thing that neem trees can't handle are soggy, waterlogged soils. The plant roots need to breathe, so don't try growing neem trees in swamps.

http://www.dw.de/using-neem-trees-to-combat-desertification/a-17016776

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It depends on soil properties and climatic condtions, 350-550 mm rainfall is sufficient  

Good information shared, thank you