I have seen little evidence from actual patients though who would report feeling better in a 12-24 months' time frame. Typically ME/CFS patients respond fairly quickly to a treatment or they don't at all at least based on my own experience and the posts I have read on different forums.
There are of course exceptions like if you are let's say heavily depleted in a mineral, it could theoretically take quite a while and say, if you want to treat a chronic infection, it could take a long time to get rid of it completely.
My understanding though when it comes to methylation or more specifically B12 treatment is it can take time to heal neurologically, but maybe it is dependant on the protocol and dosing, some people seem to be able to handle high frequent doses of B12 while others seem to have to take it slowly.
I think Greg's protocol is a reasonably slow one, where you want to start with cofactors first for a while, 3-4 weeks then introduce B12 slowly, weekly to fortnightly, maybe dependant on how you respond then build up to more frequent.
Has taken me years to get in the groove of getting a protocol right and feeling comfortable with it, glutathione this year I think has been quite helpful, and now the higher doses of B2 which I had no idea about might be interesting, especially as someone with MAO-A -/-.
I have read a lot of stories over the years of people giving up pretty quickly as well which I think is a mistake, something you need to keep tinkering with and even different brands or forms. But I also think it is important to have your personal genetic data, so you have a much more solid idea of what pathways you should be targeting.