This is a bit complicated. Not all data is available directly, but it can be found like this: we know the StuK40 was derived from PaK40, which has rate of fire listed as 14 rounds per minute (source). Since vehicle mounting is less convenient for crew to work with (confined space of the armored hull doesn’t make loading process any faster), it’s reasonable to assume slightly lower rate of fire for StuK40 and KwK40. How much lower is anyone’s guess (unless it’s specified somewhere), and most probably experienced crews can reload the gun faster.
Also keep in mind that listed fire rate is for ideal conditions, when aim doesn’t need correction between shots. So it’s very unlikely any tank with manual loading will achieve rate of fire higher than 10 rounds per minute (usually lower, sometimes as low as 2-3 rpm for IS-2).
So, all of that considered, our stug is set to fire about 9 rounds per minute (slightly more than that, more like 9.3).