It takes less than five minutes for the respawn of iron beetles along with the few wolf spiders. However, sometimes one of these wolf spiders chases a player to the navrey's entrance, in which case, I had to kill the entire spawn to make iron beetles spawn again. RNG doesnt care for me much either, so I gather at most 5 scales an hour.
I tried to get undead gargoyle medallions (and some players also gave them for free, which I'm grateful). I collected five sets of eggs for Medusa to get the medallions and I end up with none. Tried several times more but one and maybe two undead gargoyles spawn on Medusa during those too.
Stranger wrote on his guide in stratics that:
(Fun Fact!: I had to enter Medusa's lair 15 times and kill aproximently 32 undead gargoyles to get the 10 horns.)
They're not hard to gather but it takes way too much time. I need either 150 undamaged iron beetle scales or 75 undead gargoyle horns to advance to the next Tier, could only collected 50 scales and 3 horns so far.