FAST_START_IO_TARGET limits the number of blocks that needs instance recovery based on your system I/O. To set the recovery time to approximately 5 minutes, would require you to answer how much I/Os your system generates in around 5 minutes.

Best thing is that you test your recovery process each time by changing the value of this parameter. Smaller the value of FAST_START_IO_TARGET , Faster will be the recovery time. This feature is being depreciated in future versions in favour of FAST_START_MTTR_TARGET( from 9i onwards) which allows you to specify the recovery time in seconds as opposed to I/Os.