It depends on the type of samples you are working with. The purpose of cleaning the column is to remove contaminants that build up on the column from your samples. As such, injecting clean samples such as standards mean that the column will rarely need cleaning. On the other hand, biological or physiological samples have very complex matrices and adsorbed compounds can build up on the column more frequently.

A rule of thumb is to wash the column with 50/50 methanol/DI water every 10 injections for these types of samples. You can tell when the column needs cleaning by when the run-to-run retention time repeatability begins to be affected. There are several things you can do to help keep the column clean, including use of guard columns and also having 50% methanol or isopropanol in the mobile phase A solvent for Aqueous Normal Phase methods.