I believe that the half-life of LSD is around 5 hours and fully leaves your system after 24 hours or so (depending on body weight and age among other things). Also, you are taking a very small dose which is approximately 10% of the usual dose and is aimed to produce effects that are sub-perceptual. I have heard about these 'flashbacks' from full doses, never microdoses; however there is not much information or research on the matter, nor have I experienced any of these effects before so I cannot comment on it.