My observation is that even when I'm highly motivated I shouldn't go over the scheduled time, otherwise my energy to work will drop and with it, motivation.
It's counter-intuitive to say the least.
Perhaps there's something about sitting down for extended periods of time that causes this effect.
What works for me is in between. I use both motivation and determination to achieve my goals, trying to use as little determination as is feasible. When I feel unmotivated, I don't try to fight it at first. If there's something else that's equally urgent but I am more motivated to do, I'll do that. If I'm just feeling sluggish overall, I'll go eat lunch or do a workout or drink a coffee or something. And once I don't have a choice but to will myself to do that thing, then I'll do that. Much of the time I don't need to because "resetting" myself will get me to that state of mind.
Through trial and error I've settled on 30min work and 20min(!) of rest.
I have to force myself to stop working when my timer is up, otherwise my productivity that day goes down.
On one hand it's just a somewhat smarter way to eat frogs, on the other it doesn't feel so.
I suppose half of the problem is not knowing what amount of frog is too much for you.