Converge of Fan so slow?

Nepal

Hi all,
I'm simulating a fan located in a free space (just like a fan used in house), model description:
1. fan with 8 blades, I just mesh one blade and define periodic interface.
2. the interface between fan and ambient is "Frozen Rotor"
3. rotating at 2000 RPM, fan diameter 12 inch
4. no initial speed defined . The outside ambient 6 sides are all "opening".
5. Advection scheme=upwind, Turbelence model=SST
but the run time convergence curve will become almost horizontal at 1e-2 at iteration 50 , and will last the same situation to over iteration 300. Is it normal for fan simulation?Does it need thousands iterations to converge?
<font color="Blue">
Note: As I stop it at iteration 310 and view the result, the flow stream line and vector looks resonable</font>

Thanks.