I've not yet worked with nitro, but for CO2 there are many factors including temperature of your fridge, length of the hose from keg to tap, time on the gas and pressure. My method has been to place the beer at 48* at 12 PSI for a week and then serve at 10PSI. I find that slow carbing builds the levels needed and gives the beer a chance to condition.
there are multiple tables
online for carbing your beer at the correct pressure. Most breweries use a device to check CO2 levels before kegging or bottling. over carbing can be just as bad for your beer as undercarbing. It's true you can get your beer carbed early with the tricks like the one above, but if you take your time and do it right you'll be surprised how much better a beer can be. just my $.02