I concur that 3 hours would be much better than 6 hours; unfortunately it is far more common than you think for Chinese to schedule such unreasonably long English training classes. Summer camps often cram English on 5 hours a day, 6 days a week and this for a whole month.

And keep in mind another angle: Your students probably didn't volunteer for this class: They were coerced or enticed to sign up in their own interest: Keep your job or else. Get promoted and get a pay raise if you sign up...

All are "intermediate level"? That's that huge swath between those tiny extremes, the zero English speakers and the suspected bilinguals. COvers a wide area of competency in English...

The motivation of students often takes a back seat when they have to sacrifice their time off; in this case I wonder whether they usually have Saturday off; maybe they work on half the day? In that case they might feel relieved at having English class instead of their office routine.

BUt with "intermediate" learners you must be prepared to work on a large number of problem areas: Phonetics (can they read old English words which they have for ages mispronounced with the help of phonetic symbols? Can they learn something new, such as intonation?
Almost all intermediate learners need a lot of listening training. In this context you might have to revisit some grammar aspects with them. Will they be up to your expectations in terms of understanding your instructions in English?

I guess your customer will have made a plan on how to break the 6 hour class into smaller units.