We leave them open so you can decide for yourself exactly what times each slot represents. For example some people learn more effectively in the morning so will want longer revision slots there while others prefer the late evening - it's up to you.
I've decided that they are 2 hour blocks. Eg. Early AM is 8-10, Late AM is 10-12, etc. this way I can split it up into smaller chunks, maybe 15min, 20min, 30min - after which I take a small break to get a drink or stretch my legs etc.
Plus, I don't think I could get up pre-8 AM, and this way it finished at 8PM, leaving me time to socialize etc =)