It depends where you currently are with your revision. For example, do you understand the content for all of your courses yet?
Rather than setting a number of hours a day to work, I feel it's better to plan out what content you need to cover each day. This involves taking the time to figure out what you actually need to revise in advance and structuring in a format that ensures you have time to get through all of it.
I've Higher Biology and Chemistry textbooks sat on my desk right now so if I was to plan out my week I might go:
Monday
Biology: (1) Science of Food Production (2) Photosynthesis and Energy Transfer (3) Breeding Plants and Animals for Food
English: memorise quotes, practice essay
Tuesday:
Maths: practice questions
Chemistry: (1) Fuels (2) Hydrocarbons (3) Alcohols, Aldehydes, and Ketones
Modern Studies: Africa topic content
Wednesday:
Biology: (1) Crop Protection (2) Inter-relationships and dependence (3) Social behaviour
English: close reading practice
Thursday:
Maths: practice questions
Chemistry: (1) Carboxylic Acids and Esters (2) Polymers (3) Natural Products
Modern Studies: UK content
This will feel like too much in a day for some people, and too little in a day for others. Take a good look at the content of each of your five Highers and figure out what you are confident with and what you aren't. Outline what you want to cover for each class, arrange it in a schedule that works for you, then jump right in.
This is just my personal preference on how to tackle revision. I find it clearer to just work for as long as I'm able to and until I hit my goals for the day rather than watching the clock.