Which is the better training protocol: 2x20' at FTP with 10' Rest Between Intervals (RBI) or 4x10' at FTP with 5' RBI?
Are there benefits to doing longer intervals, or is the total time at spent at FTP really the most important measure?
It is honestly personal. What works for some people may not work for others. You need to find what works for you - and it can take time.
I don't really do many longer intervals such as 2x20, but I can see their benefit. They just seem to drawn out for my liking. My bread and butter workout has been 4x10. I know a friend of mine doesn't even do FTP workouts but instead does 1 minute intervals with a 5 minute recovery.
But generally speaking, when I am in one of my FTP build weeks I try to spend between 90-120minutes at threshold during the week (I only include intervals of 8 minutes or more in this number).