So many variables involved. Around 15% moisture. But, it depends on type of hay/grass, location, how it is baled, time of day it is baled, etc. With experience, you are better able to decide when to bale by feeling and twisting the hay. If the stems snap like uncooked spaghetti, it is past time to bale. If the stems bend like a rope, it is too wet to bale. If the hay has been conditioned, it will dry quicker and be ready to bale sooner. If it is teddered at the right time, it will dry quicker. Amount of sun and humidity make a big difference. A moisture tester measures surface moisture which is influenced by relative humidity of the air. If the stem still has too much moisture, the tester may read ok, but the moisture will rise after it is baled due to the moisture coming out of the stems. If it is sorghum/sudan, the stem will have lots of moisture and will benefit from a conditioner with rollers that crush the stem as opposed to an impeller type conditioner that beats the wax layer off the stem and is more effective on grass hay.
But good quality hay is not ready to bale the next day after cutting. Poor quality over mature hay maybe. That's the way it is in my part of the country. Hay will be wetter as soon as it is baled. More so the next day. Risk of mold is high if too wet. Risk of fire as well.
High yielding good quality spring hay here is going to need about 5 days of curing - depending on all the variables. Lots of people bale too wet, especially if they are selling the hay.