My rule, based on actual experience:
Standard action to fling the flour in a single 5' square.
Full-round action to fling the flour in the eight 5' squares around you.
The flinger and everyone in the squares you fling into must make a Fort save DC 12 or choke and cough for one round. No action other than moving away can be performed while choking and coughing.
The flour stays "potent" (in the air) for one full round after being flung. Anyone staying in, coming into, or passing through the effected squares (including the flinger's) during that round will be affected by the choking and coughing (and *light* powdering).
It ain't easy being a geek.