How long it takes for a substance to get out of your hair ?

It depends: hair grows at 0.5 inches per month. So for each month you do not use the substance your hair will grow 0.5 inches. It will depend on how short you cut your hair how long it will be before the substance is out of your hair. note that if the hair is not cut to a level short enough to get to the hair grown after use has stopped the substance will still show.

Depends: Depends on the substance you speak of. Gum, glue, paint? Need more detail about the aforementioned "substance."