From my personal experience, my team has always found that the cooldown was 12-16 days. It certainly is difficult to figure out which spot is available but if you're consistently checking, you'll find one eventually and figure out the cooldown for each location. It also helps to team up with other people checking so they can share if a boss was found, killed, or despawned.
I've also noticed that if more than 1 Rotworm Queen was killed in 1 day, it throws off the timer for some tracking sites. It will say 1 is on cooldown and 3 are available when actuality 2 are on cool down. This applies to other bosses with more than 1 spawn point as well.