Use a microscope with an eyepiece graticule to measure the lengths of individual fibers. You do not have to measure all the individual hairs - you can use random sampling. Random sampling means you measure say 100 hairs at random from the slides. You can estimate the rate of hair growth by adding up all the hair lengths you measure and dividing by the number of hairs measured. This should give you the average length of the hairs.
More advanced students can work out the standard deviation and standard error for the data.
Beard growth rate varies from person to person and depends on their health and age. The rate can be anything up to 0.4mm a day. There are a couple of different ways you can plot the measurements out on a chart. You could group the measurements into sets. Count the number of fibers in that are 0-49 micrometers in length and so on.