What is the best string wax to use on FF strings?  How often should I wax it?

From BCY website

Question: Is it necessary to use "special" wax with certain types of bowstring material?
Answer: Not really. There are four main reasons for using wax on a bowstring.

1. To lubricate the fibers and prevent "fiber to fiber" abrasion
2. To help keep the "bundle" of strands together
3. To maintain and extend the life of the string
4. To help prevent water absorption

Bowstring material manufacturers normally apply a generous amount of synthetic wax in liquid form, which insures that from its origin the bowstring material is well lubricated inside and out. It is this process which gives the material its basic protection from fiber to fiber abrasion and helps keep the
bundle of strands together. Generally the type of wax used would be a good quality standard "tacky" wax. At this stage it would be unusual for the wax to include silicone which is very slick and, particularly in the case of Flemish strings, makes it difficult to convert the bowstring material into a finished bowstring. However, after the string is made, it is recommended that the archer look for a "maintenance" wax that contains some silicone because a wax blended with silicone penetrates the string material very well and keeps the inside fibers lubricated as well as the outside. Wax can be applied with fingers, with a cloth, or with a chamois leather. It should be applied frequently and rubbed well into the string.

You can melt 1 pound of beeswax and 1 toilet wax ring, mix them together and pour it in tiny paper cups. Makes excellent string wax and will make enough to last for years.

BCY's "X-Wax" is great.  A tube is cheap and lasts a long time.

X-wax +1