When should I start collecting fractions when I run column chromatography?

When running column chromatography, it is ideal to start collecting fractions when one is able to see a line corresponding to the polar solvent used to dissolve the sample. If the line from the polar solvent is not visible, mark the starting solvent level by drawing on the exterior of the column. A second mark should be made on the column beneath the first corresponding to two thirds of the silica length. When the solvent falls below the second mark, one should begin to collect fractions.