Does your "best effort" at practicing the piano mean your maximum output possible? If so this is simple to define the amount of time you need to be in front of the piano, it is when you cannot physically sit anymore because your backside is numb and your fingers are burning with fatigue, but that is not a balanced way to live life!
Still practicing for hours on end does not necessarily mean you get through a lot of work, there is usually a peak that we reach which if passed we simply learn at a slower rate with more time, or not even retain anything at all! I find this occurs in my students at different rates, some even after 5 minutes of focusing only on piano they are exhausted!
"What gets written, gets done" is something I keep telling students of mine, when you observe something in the sheet music that helps you memorise or control a passage write it down on the sheet music. Do not leave it in your head because things in our heads vanish without us even knowing it. Write it down and you will constantly be reminded of it, even if you take the sheet music out in 10 years time to relearn it.
In my mind the "best effort" at practicing the piano is maintaining a good bar per unit time rate and constantly aiming to push that higher. That is very simplistic but difficult to monitor and control. Many people get distracted with movements or sounds they cannot controll 100% and thus waste a lot of time trying to get it right without a plan or practice tactic (this is where a good teacher comes in handy). Some get distracted with the telephone, family, friends thoughts about the day etc. These two factors usually usually without knowing, make us choose to practice longer and harder instead of smarter.
I dont even care to ask students how many minutes/hours they practice, I measure their achievements on what they can actually show me. There is no point practicing for x hours and get no where! Time means nothing unless there is work to show for it!