Some reactions require that temperatures of 70–100 °C to be maintained for periods of time as long as 24 hours. Why is this so?
Quite simply, reactions happen faster at higher temperatures (because who likes waiting), and some may not proceed at all until heated. From a statistical viewpoint, you are giving more reactant molecules more energy, which thereby increases the number of collisions, which in turn increases the number of successful collisions (those which produce product). A typical way to attain such conditions is to place a reaction under reflux.
While I think both of the other answers are good, I want to comment on two practical constraints that yield long reaction times:
- Solvent choice - We perform a lot of chemical reactions in solvents. This allows easy mixing and usually facilitates separations and purification steps after reaction. On the other hand, if some reaction requires a high activation energy barrier, we cannot always simply raise the temperature or most practical solvents will evaporate (e.g., we can't run most reactions at 300-400 °C even if it would minimize reaction times).
- Human timescale - Even if some reactions might complete sooner, it's practically much easier to run a reaction for ~24 hours - you set up the reaction in the morning and then it's done the next day. It's neither safe, nor practical for someone to come in at 2 AM to shut down a reaction.
Long hold times are needed for one of two reasons:
(1) Mass Diffusion limitation - For example, in a drying reaction moisture needs time to diffuse through a sample to the surface before evaporating.
(2) Temperature limitation - For example, in a conventional drying reaction, the part of a sample nearest to a heating element heats first. The heat needs time to travel through a sample without getting any part of the sample too hot. If part of the sample gets too hot an undesirable reaction like combustion may occur.