It could be a fraction of a second or 100 years before a measurable amount of products is formed. It is still spontaneous. Natural gas continues to burn once it is lit. Water runs downhill. Hydrogen burns in oxygen to form water.
Sodium reacts with water.
An early criterion for predicting whether or not a process was spontaneous was that it had to exothermic. The sign of H is not enough information to predict accurately whether or not a process is spontaneous. Entropy, symbol is S, is a thermodynamic quantity that is a measure of the randomness or disorder in s system. Entropy describes the extent to which atoms, molecules, or ions are distributed in a disorderly fashion in a given region of space. For mixing of two ideal gases, after a certain amount of time, each gas will be evenly distributed between the two flasks.