Let me preface this by saying that I'm not putting down or insulting anyone who does things differently than me, as in most cases, no harm is done by longer acclimations.
Having said that...
Nothing but nothing needs to be acclimated for hours with the possible exception of certain echinoderms and cephalopods. People acclimate longer for the most part because they lack the experience that tells them this isn't necessary, and as I've said, for the most part no harm is done in erring favor of the animal. I've been at this for over 20 years however, and everything, including every species of Tridacnid need only be floated to equalize temperature, accompanied by a "slow" introduction of tank water, over a 30 minute period or so. That is, unless the temperature and/or salinity difference is drastic between your tank and the tank from which the clam was previously being kept. Then you can push it to an hour.
I have yet to lose an clam or coral due to acclimation problems. Tridacnids quite frankly are not that fragile. Rarely have I come across an easier, more trouble free animal to keep in a reef aquarium.
I've read posts about how to acclimate cleaner shrimp for instance, and inevitably someone will post recommending a drip or something similar. In reality, you net them out of the bag and sidearm them into the tank..done. He skips along, ricochets off the return pipe, settles to the bottom and lives his happy little life.
There are a few species of fish I'd acclimate for longer than 30 or 40 minutes, but not many.
I think people have a bad experience, then overcompensate...or read somewhere about 3 hour acclimations and this becomes their doctrine.
I'm not recommending beginners adapt all my practices...just food for though.
Best Regards
Jim