I have been on Mirtazapine (Remeron) for the last 4-5 years. I have never gone off this anti-depressant, so unfortunately I have no answer to your question regarding withdrawal.
I would suggest that you post your question over on the "Medications and Therapy" section of the forum. There are some wonderful individuals where, such as Insights (Ian) and Abraham (Steve), who might be able to respond to your question.
The very best to you, EO!... Chuck