It may not be a good idea to get off of psychotropic medication(s) during the change of seasons (especially if someone has a diagnosis of bipolar which, requires medication in order to keep symptoms at least at a base line)
For SAD, what may be helpful is the use of a blue spectrum light. One company that had done a lot of research on their product is GloLite. The best time to start using the blue light is at the end of summer and into the spring. This helps regulate the 24 h/sleep rhythm and this helps with the depression that manifest in the fall in people with SAD.
You may want to consult with the prescribing physician about their take on getting off of the effexor by tapering you off of it (yet, I think that medication and a blue light may be effective when used together) If a person with bipolar disorder is not medicated, they can still lapse into depression or become manic irregardless of the seasonal changes.
it generally takes as long as for the medication to start work initially about 4 weeks (consult with the prescribing MD to see what they thin is better)
People can have withdrawal symtoms if they stop the meds too fast.