Separation starts immediately and proceeds at its own pace. On the common the paints they sell for painting houses, I would expect separation to be very slow, and not be significant after only 5 hours.
What is happening is the resin and pigments (heavy) are settling toward the bottom while solvent (stuff that vaporizes away) settles toward the top. So your paint will be a little bit scant on pigment, but not enough to matter, I wouldn't think.
As a counterexample, "cold galvanizing paint" has resin that is 90% zinc metal, and it's heavy and starts separating in seconds! Constant stirring in the pot is essential or you finish the job with a huge glob of zinc peanut butter in the bottom of the pot, and what you painted was mostly thinner lol.
Another thing that happens is the various pigments weigh different weights, and will separate from each other given a long time (months or years). But you seem like a person who doesn't keep paint, so that's fine.
Modern paint (read: for houses) is almost all "latex" and has very low VOCs (toxic solvents that cause smog). Since it's not toxic, unfortunately it can go rancid. If used, it creates a rancid room (mold and bacteria). So it's actually wise to not keep latex.
Note that paint should not be dumped down the toilet where it damages the sewage treatment process, or thrown in the trash where the can will burst open, making a mess for garbage handlers (my mess up the day of trash handlers, I've seen more than one line down the street from cans burst open inside the garbage truck). If it makes it to the landfill, it will leach into ground water. It's non-toxic, but not that non-toxic. To dispose of old paint, paint something you don't want and throw it away. This complies with environmental rules as well, and works with any paint, including the extremely toxic 2-part epoxies and urethanes (e.g. auto paint).