Tar dries slowly by losing volatile parts and forming a more stable surface, but many tar coatings do not dry into a hard modern-film finish.
Drying is a slightly misleading word for tar. Water-based paint dries because water leaves. Tar settles through a more complex mix of evaporation, oxidation, absorption into the surface, and weathering. The result may be dry to the touch without being fully hard underneath.
A thin tar coat on dry, absorbent timber can become touch-dry much sooner than a thick coat on smooth or damp material. Air movement, warmth, and surface area all help. Enclosed spaces, cold weather, and heavy application slow the process.
The smell of tar often continues after the surface feels dry. That is because small amounts of volatile material can keep leaving the coating. Warm weather may bring the smell back, especially from fresh work or thick deposits in cracks.
A dry tar surface should be judged by use. If it no longer marks easily, sheds water, and remains bonded, it may be dry enough for its purpose. If it smears, lifts, or collects dirt aggressively, it has not settled well.