You can get as much work done in 1 hour of intentioned, focused activity as you'll get in 10 hours of working while checking facebook/twitter/text messages/youtube tutorials/webforums or otherwise distracting yourself while working. Humans cannot multitask, the concept is a myth. We simply divide our attention between two tasks, which lowers our efficacy at both. So it's not even a matter of focusing on results over time or time over results, but focusing on the task at hand that is important. Unplug your internet every time you want to produce or use some sort of blocking program like AntiSocial or Focus - do whatever you can to prevent yourself from succumbing to temptation and WORK!
That being said, you probably shouldn't shoot for a specific timeframe of production unless you're working on a restricted schedule, because you'll never know how long any one particular production session will take and it's easier to be productive when you're not just wandering aimlessly through your DAW. Personally, I work with what I call "Fix Sheets" - A written list of everything that I think needs to be added, removed, changed, or fixed in my current project file.
I start my production session by listening to the song from start to finish, and writing down little notes based on how I think the song should be fixed. "Add shakers to second half of drop", "change lead synth melody to jump around less", "move white noise lower in mix", etc. I don't make any changes until I get to the end - and I either work in the order they were found or in order of what I feel is most important to least important. If I find something else I need to fix while I'm working, I add it to the list instead of changing my focus and allowing any distractions in. Every time I finish everything I've written down, I go back to the beginning and repeat the process until I just can't think of anything else that day, which means I'm probably at mental capacity and are hitting those diminishing returns you're talking about. If I'm not at mental capacity, and I've already done due diligence in referencing the track against what's commercially available, I can take that as a sign to just consider the song done and move on.
This can be 3 hours, 6 hours, or 12 hours, depending upon how much I identify, how close. I do try to stymie things by using the Pomodoro technique to let my brain decompress, but what I've found is the more comfortable I get with making music, the longer I can go in each individual session. So comfort in your own skills might play a part!