Interesting. Do you have a source on this? I can't seem to find any detailed data on this. Only some suggestions that since they're lipophilic, and are found in higher concentrations in brain tissue, they might accumulate there. I can't find any research analyzing whether they do accumulate in brain tissue and how long they stay there.
If it was weeks, they would accumulate to very toxic levels in anyone who smokes or has a few cups of coffee a day.