soil? soilless? impacts what you do.
soilless -- reduce fertilizer concentration slightly for seedlings and quickly ramp up to 1.3-1.5EC concentration... you fertilize every single time in soilles. you make sure there's 10% runoff and it ensures a stable, predictable concentration of nutes in substrate at all times.
soil -- bit more complicated, but has some conveniences too. Soil comes with a charge... you may not need to add anything but water for upto 60 days if it is "hot" soil. hot just means high concentraion of nutes to start. As the soil's initial charge is used up, you ramp up fertilizer concentration over time... i'd suggest fertilizing every other or every third irrigation. the frequency is an option you can use to adjust how much you give the plant... along with simply mixing up different concentrations of course... but as you learn to ride this bike you may need to mix both together to find a happy zone.
so, if it's soil and this is a seedling, i'd wait until i see the cotyledons start to fade -- this is a sign the substrate is not quite providing all it needs. Take notes of how long this takes and "next" time pre-empt it with a fertigation even if you don't see the coty's change -- this also assumes you use the exact same soil composition/brand/etc.
start at 1/4 or 1/2 and go up from tehre when it is time... ever 2nd? every 3rd? whatever keeps it happy. You'll probably feed at a higher EC than soilless.
When to irrigate... First, you water entire pot. (some exceptions if very tiny plant in a very large pot, otherwise water entirity.) You wait until top 1" is dry and repeat (feel weight of pot at this point, this is a better trigger to use)... a little runoff ensures you watered entire pot... if you get the volume dialed in, you can reduce that to a minimal amount.
If you water at same weight, it requires same volume.
You do not pick a volume.. the variables of your soil composition and size of pot determines how much you need to give. Don't try to pick this number out of a hat... you learn it retroactively after your first irrigation...
this habit promotes proper root growth.. if you don't water entire pot, dry pockets can wreak havoc over time wiwth concentrations levels of nutes in your pot... Also, superficial watering will promote superficial roots... you don't want that... roots turn toward greater moisture.. no thinking.. just a simple reaction to environment. you want those roots driving down, not up.