Thats a really good question especially at this point in time, the 'psychobabble crowd' seems to rely on what I suspect are false statistics that a large percentage say 30 or 40 % can recover entirely within a few years if you follow their CBT, GET whatever.
I heard stories of people saying they recovered or had relapses and its often described in terms of like '75% back to normal', I don't quite get what they mean by that though since you either have PEM or you don't,
perhaps the best person to ask would be a specialist CFS doctor that oversees large numbers of patients and often refers them to researchers, theres a few in the US but I forgot the names, they would diagnose people using the strictest possible criteria, and if people were recovering completely after a few years, they would be the ones that would notice.