We consulted David directly on this question as well, and here are some more specific thoughts on long term use of herbs, including Valerian:
For most long term daily use of herbs, it is best to be done under the direction and with the support of a professionally trained herbal practitioner, especially for something as variable for each individual as Fibomyalgia.
If using Valerian at a lower dose (like 2-3 mL / day of 1:3) it is best to take it daily for 4-6 weeks non-stop to accrue benefit, but at that point it may be best to take a break from it, switching to another nervine like Skullcap (Scutellaria lateriflora) for 2-3 weeks before returning to Valerian for a month or so. The nervous system does well with tonics for weeks or months at a time (like Skullcap) but not necessarily so well with the less tonic but more sedative nervines (like Valerian).
However, this is very individual and the most beneficial way forward is likely to book a consult with a professional, to be able to find herbs that might support at a more specific, deep and safe level for that person.
I hope this is of some more clarification and help,