Terri sent us her answer to this question.

After we've counted queuing for the rides, they have 4/5 of the day left.

After we've counted the rides themselves, they have (1/4)x(4/5)=1/5 of the day left.

After we've counted lunch, they have (1/2)x(1/5)=1/10 of the day left.

After we've counted the cinema, they have (1/3)x(1/10)=1/30 of the day to spend in the gift shop.

But this is 10 minutes. So 1/30 of the day was 10 minutes, so the whole time must have been 30x10=300 minutes, which is 5 hours.