I'm on mirtazapine 15mg i'm noticing i'm having to urinate more often and frequently... willl this side effect go with time?

Maybe in part: Remeron (mirtazapine) is a medication with effects on two neurotransmitter systems. The second, "adrenergic", effect does cause the side effect you describe (often along with, say, dry mouth, constipation, a bit of blurred vision). Your body is likely to adjust some to having it on board, but probably not entirely. It's a trade off that you should discuss with your prescriber.

Remeron (mirtazapine): Likely so. Urinary frequency with this drug is usually mild and reduces with time in my experience.