I think it depends on your special issuance condition and how many specialists are involved in your review board. If you have more than one issue then more people will need to give the file a look.
I think the fact that it's under review with no additional required documents requested suggests you are close to an answer?
I was told recently that it was taking about 3 months on average. However Christmas/New Year holidays and leaves will probably add a month. Are you calling them every other week or so?