The effects on memory are related to the time since you smoked and are most noticeable after 1-3 hours. Basically, you get stimulated at first (during which time you may actually have increased creative problem solving) and eventually you get tired or "burned out". It feels a lot like low blood sugar which is why some people over eat. If you smoke a lot these effects can linger as a sort of hang over. If you smoke every day for years, it might last a few days.

Speaking for myself. If smoking regularly yes, there's a decline, even 12 hours after the high has gone. The more I spread things out however, the greater the benefits seem to be, both during and after. Every couple of weeks would likely be most ideal.