I'm not sure we know how percent efficient it is. I had a look but the limited studies seem to use 3x5g daily for CFS and other conditions. There are a load of other studies for cardiac patients and some studies on rats and quite a good write up here referencing the papers.
https://examine.com/supplements/d-ribose/
If the theory is correct then you would expect to need to constantly dose and the required dose to vary depending upon level of activity and severity. The only study on CFS which used self reported measures, has a potential COI and didn't use a placebo so this isn't much help (sorry)
"D-Ribose is thought to interact with fibromyalgia based on the knowledge that there are perturbed skeletal muscle energetics in subjects with fibromyalgia,
[14] most notably reduced ATP levels (of which D-ribose is a component),
[15]and how D-ribose in high doses can influence this tissue in athletes.
[5] One case study has noted that 5g of D-ribose taken twice daily alongside other medication in a subject with fibromyalgia greatly reduced symptoms, which returned after one week of supplement cessation.
[16]
In subjects with fibromyalgia and/or
chronic fatigue syndrome (CFS) who given D-ribose at 15g a day (three divided doses) for just under three weeks, supplementation was associated with subjective improvments in energy, sleep, and well-being with an increased pain threshold.
[17] No placebo control was used in this study and there may be a conflict of interest (product supplied by a company of which produces D-ribose and employs one author).
[17]"
