How many lures are needed for constant-catching?

I think lures spawn something once every 3 minutes, so five at once would mean a new pokemon appearing every 36 seconds, plus natural spawns. Most pokemon ought to be catchable within 30 seconds or so, but some take longer, so five lures sounds about right. Any more than that and I think you'd have trouble keeping up.

If frugal, 3.
If rich, all.

I have more lures than I can use with the 30 min limit but now, taking advantage of the final hours of the event, I've used them on all 9 stops on this 1 km loop trail and the Plus and I are constantly catching and spinning stops /gym discs. I wish they normally lasted longer than 30 min....