Does the dissociation of serotonin receptors and their ligands really take hours?
That paper is describing the binding between the 5HT3 receptor and some high-affinity ligand, not serotonin.

Numbers of serotonin probably vary according to the precise receptor (species, subunits, etc), but for a ballpark figure this source says 1.7/s, giving half time of ~400 ms. That estimate is based on solution changes, so I'd say it's an upper bound.

Remember from the other questions you have asked and I have answered, serotonin is a slower-acting neurotransmitter, and clearance of serotonin is not as fast as for other, faster-acting neurotransmitters. However you are correct that your initial calculation of a ~4-hour dissociation time is unreasonable.