How long would it take to cover a single paddock using more standard techniques (manned aerial or ground-based)?

Our current self propelled sprayer can cover roughly 70ha an hour. 36m boom travelling between 18 and 25km/hr. Water flow rate also plays a part in how much you can cover.

Is a week too slow? How often do crops need dusting?

Yes, a week is far to slow. We need to be able to spray multiple paddocks a day, and respond to weather events quickly. I.e you may to want to spray a few thousand hectares in anticipation, or right after a rainfall event.

at 15k versus the cost of a helicopter or plane i think the solution is to simply buy a fleet of however many you need.

So our current sprayer covers ~70ha/hr. That means we need a fleet of 18 drones to match it. That's 270k + the associated inferstructure costs. If there not fully autonomous you'll also need to pay someone $30-$40/hr to stand around an continually refill, program and move them from paddock to paddock. As well as ensure there aren't miss sprays, crashes, spray drift onto neighbour paddocks etc. However given the flight time is <20 minutes you'll need far more than 1 person for that, maybe 2 or 3.

We can hire a spray plane for $12/ha, or a contract sprayer (self propelled boom) for slightly less than that.

This introduces a coordination problem that is easy in principle (just like concurrency on a multicore machine is easy in principle).