Natural Gas Tankless water heater condensate volume
Humidity will have little to no effect. The condensate is a combustion byproduct, and will be determined almost entirely by how much the heater runs - when the heater is running, expect pretty much the rated output. When the heater is idle, expect none. As such, volume per day will depend on your hot water usage.
What, if any, treatment is required before irrigation use will depend in part on your local conditions. I know some desert soils are very alkaline and might neutralize acidic condensate quite well themselves. Beyond that, some experimentation is likely called for. A small reed bed filter (see virtually any permaculture reference book/website) or the like might be beneficial before feeding to more sensitive plants - condensate is often a bit weird due to minor combustion byproducts other than water.