I've farmed some items for fun (ingredients of greedy ring), and got them after 6 Prate invasions, getting coin gun took 8-9 more invasions.
Currently farming RoD, the biggest fear is that i leave the world when it's dropped and i'm not collected it because i wasn't attentive to the items laying in "hallowed cave area" (in brackets because it's looking too bad to be called like that)