There are no rules for the frequency at which you apply carb heat. There are simply conditions under which you are more and less likely to suffer. If the OAT is -40, you can probably safely assume that your Carb will not ice. If relative humidity is 5%, you can probably make the same assumption.
You should develop an understanding of the risks associated with the conditions in which you are flying, and apply carb heat as required, whether that is every 5 minutes or even more frequently.
When you do apply carb heat, as posted above, 15 seconds is probably too short a period of use. The risk is that you start to melt the ice sufficiently for the water to run back out of the way of the direct heat flow, where it re-freezes. Continued use of carb heat in that manner runs the risk of causing ice blockages that will not shift at all.
Some engine and aircraft combinations are more prone to icing than others, and are therefore more touchy if Carb heat is not used appropriatly.
Short answer... there is no single answer to the question. You need to know your aircraft and the conditions in which you are flying.