Well, it really does depend on what you want to test and what the question is.
I like to test for CO2 and O2 right as it comes out.
This tells me what I'm adding to my tank and if the CO2 is good enough from my CO2 system(you see a large increase in growth after waters very often, this means I can likely stand to add more CO2)
Some let it sit for other reasons.
Generally, most let it sit for 24 hours(KH/GH/NO3/PO4 etc).