STM32F091 Flash status SFR timing question during erare

[OSedl.1]

I'm working on a bootloader which needs to erase and program the Flash. The erase seems to be 'working' but I have a suspicion that I'm not waiting long enough for the 'Busy' bit to go active after setting the 'Start' bit. The reference manual says I need to poll for 'Busy' to go active, Can anyone give me the timings for the Flash status register?