My "rule of thumb" is: if you can smell it, it's not fully cured. Get your nose right up on the surface and take a good sniff. Smell anything? If so, it's not ready for top coating.
This has served me well and I have (knock on wood) never had a paint system failure.
But, this is also why I go with catalyzed acrylic urethanes- they are essentially epoxy, and cure via chemical reaction, rather than evaporation of solvents. So you can lay them on thick and once the cure time has expired, it is good to go.
Conversely, something like a nitrocellulose lacquer has to evaporate the solvents and that can take weeks. I used that for sunburst finishes and tinted the colors myself. Great stuff but it takes a lot more time.