Thinking about it, even just a simple tick box for 'per hour' time limit plans, specifying that I want unused minutes to accumulate, would be brilliant - I don't think I really need to set a maximum number of hours over which they can accumulate.
So I might block twitter (or whatever distraction is my nemesis) completely from say 21:00 to 12:00 (the next day), but from 12:00 to 21:00 have a plan running that allows 5 minutes per hour, and tick the box saying 'Allow minutes to be carried over to the next hour'. My reward for staying focused on work and completely off twitter until, say, 18:00, would then be that I could use it for 30 minutes - the 5 minutes for all the hours from 12:00 to 18:00, saved up and taken together.
So I might block twitter (or whatever distraction is my nemesis) completely from say 21:00 to 12:00 (the next day), but from 12:00 to 21:00 have a plan running that allows 5 minutes per hour, and tick the box saying 'Allow minutes to be carried over to the next hour'. My reward for staying focused on work and completely off twitter until, say, 18:00, would then be that I could use it for 30 minutes - the 5 minutes for all the hours from 12:00 to 18:00, saved up and taken together.