I’ve experimented with forced time ratio’s.
But that was mainly to get over the terrible habit of being addicted to distractions.
Now that’s out the way, I tend to just work for a minimum of an hour. Sometimes I’ll get an interesting problem and tackle it for longer, other times I won’t.
Usually I flounder by the 90 minute mark and have to take a break anyway.
Still doesn’t consist of distractions though, typically it’s as simple as grabbing a cup of tea from the kitchen.
You’ll have good days and bad days. What works for me is recording what I did when and leaving the future open to be used up.