Tacky Paint

The question is a bit broad to answer easily. What type of paint are you writing about?

It is not considered good practice to paint over paint films that have not had a chance to go through the drying process. This creates a situation where the superimposed paint will dry far quicker than the underlayer. This makes a situation where the under layer continues to move while the upper layer become hard and eventually brittle. The upper layer really can do little other than crack in this situation.
The situation is quite different when it comes to blending. It is often easiest and most effective for subtle blending if it is done when the adjacent layers are tacky. Now this is different than trying to blend two ajoining applications of paint that are thicker and have begun to skin over. Attempts to blend at this point are likely to break the skin and cause a general mess.

Six months is not necessary. The general rule of thumb is that the paint should be dry enough so that you can press a finger nail into it and not leave an impression. We discuss this in our “Resources” section under “Mediums and Additives.” I have copied that paragraph here: If you are painting in successive stages and want to determine if the underlayers are sufficiently dry for overpainting, one can use the “fingernail test”: If you can press your fingernail onto the oil paint without making a dent in it, you can continue to paint atop the surface without significantly biting into the layers beneath if you avoid using too much solvent.