Hi Gwenny,
I too - like many of us - found the decline in cognitive function pretty annoying, even distressing.
Per your first post, I did some digging on the net; found a long scientific paper about the role of Thiamine (Vit B1) deficiency in alcoholics. Won't post the link, as after all, when we can't concentrate too well, it's too hard to read through the damn thing :-). But I do know from my previous rehabs as well as my latest detox at home after a relapse, that B1 is critical, in addition to B Complex. IN other words, take a separate tablet of B1 as it has a higher dose than you'll find in B Complex.
RE the timing (roughly, as everyone's different) though, I did find a very useful table via the HAMS Network website. (They're into both abstinence AND moderation, just for the record).
http://hamsnetwork.org/images/cogdysfun.JPG