IDK if it's on 20 hours but a lot of stuff is so I wouldn't be surprised. General rule of thumb is that the good stuff from boxes, packs, etc is on a 20 hour CD.

The cooldown on furnishing plans applies to looting containers like urns, wardrobes, backpacks, etc.. and doesn't apply to other sources such as monster loot, reward boxes, Azandar's research portfolios, fishing containers, etc. I've batched opened several algae packs in one go many times, and can confirm that the cooldown doesn't apply. But rng is rng. So mileage may vary.

[caveat] As far as I recall, ZOS has never acknowledged the existence of the cooldown/anti-farm mechanic. But it's widely accepted via mountains of anecdotal observation.

Another indicator that it's not on any cooldown mechanic, is that algae laden packs don't drop white furnishing plans.