2 - 4 weeks. Like most other antidepressants, Mirtazapine (brand name remeron) usually starts to result in improvement after about 2 weeks of use, with increasing benefit over the next several weeks. In my own practice, I tend not to wait much past 4 weeks if there's been no benefit at all; other psychiatrists will say to wait up to 6 or 8 weeks.
How long. You will realize the sedating properties of the med the first night; however, the antidepressant properties will probably not be felt for 2 to 4 weeks. If you are anxious, these properties generally not felt for 4 to 6 weeks, but could be sooner. An important thing to remember with this med is the lower the dose, the more sedating it is. Sounds odd, but if too sedated on 30mg, don't have your dose.