Flushing after root drench?

Do you have to flush anytime after you do a root drench with azamax or botanigard?

No only when your close to harvest 3 weeks out or if theres too much salt build up ! if your in veg ur good

If I'm in mid flowering, how soon after the drench should I flush with plain water and how water needs to move through each pot to effectively flush?

this is soil right ? if so , enough to get a good run off,although giving a heavy watering does leech it out(flush) it doesnt completely flushing is a process that needs to be done over time like flushing it 2x for the week. flush it this once then continue with your schedule it should flush out completely when you do your last flush

You dont need to flush with an application of botanigard. Botanigard is a biopesticide, its a fungus.....
http://en.wikipedia.org/wiki/Beauveria_bassiana