There is nothing that governs frequency it is a commanders program. If the program is not operating properly you should consider putting together a good solution and present it to the chain of command. Most unit struggle with running a good Remedial PT program. If you provide a good solution the command may take your idea or even put you in charge of it. Special PT should be designed for the individual and the challenge they must overcome.
AR 350-1 Appendix G d(5) states:
(5) Special physical readiness programs will be developed according to FM 7–22 and kept separate and distinct from the Army Body Composition Program, except for the exercise programs prescribed to assist Soldiers with weight control problems. Commanders should avoid placing all Soldiers that exceed body composition standards or have physical readiness problems in the same category with the expectation that more exercise will automatically result in improved body composition. Special programs will be employed for Soldiers with physical readiness problems. Separate special programs or additional PRT is not always a solution to overcome identified weaknesses. Commanders are encouraged to place Soldiers with special physical readiness needs into the regular unit PRT program whenever possible. These programs will not be used to discipline Soldiers who may have attitude problems toward physical exercise.
For more information on special PT see FM 7-22. In many cases the command outlines special PT in a policy memo
Be the Solution!
If you bring the command a reasonable and well thought out solution more than likely they will implement it. Be the change you want to inspire