Hi there! There is preliminary research showing that low doses of cannabis activate serotonergic neurons, thereby increasing serotonin—and has even been shown to exert potent antidepressant-like properties in the rat forced-swim test (FST). However, the same study showed that higher doses of cannabis actually reduced serotonergic action from baseline. http://www.jneurosci.org/content/27/43/11700.
Additionally, there is scientific and a plethora of anecdotal evidence supporting the notion that heavy cannabis use can lead to the worsening of depression. Therefore, it becomes clear that cannabis is best used at the lowest dose possible to address your depression. http://www.jneurosci.org/content/27/43/11700.
There is no dose equivalence between CBD and Lexapro. Also, CBD can be safely taken with Lexapro, simvastatin and melatonin, so there is no need to wait while weening off of Lexapro.
As an example: https://www.cbd.org/products/chocolates/18-1-chocolate - take one piece twice per day http://dosist.com/pen-bliss/ - take one to two puffs only when feeling especially depressed.
Hope this helps!