How long does nCE take to deassert the FPGA pins

Guys,

I was wondering if anyone knew how long it takes to deassert a cyclone 3 (EP3C10) when nCE is asserted high.

I have a cyclone 3 setup in AS mode. When the config prom is blank, the FPGA continually tries to configure itself. To stop this, so that our micro can take control of the AS configuration bus for programming the config chip, we assert nCE. I have noticed that it can take around 30us for the FPGA to stop trying to reconfigure and stay silent. I kinda thought this would be near instant but it seems like it waits to finish something before it tri-states its pins.

Thanks in advance

C