Turnaround times are something that could make the difference between spending an unplanned night in the woods vs leaving at a reasonable hour. The basic idea is that you should be at the furthest point of your hike at an early enough time that your hike out will end on time. The simplest way to figure this out is take your start time and desired end time. The end time may be a set time because you have to be somewhere, or it could be sunset/dark. Calculate how many hours there are between the two times and divide by 2. Add that back to your start time and that is when you should turn around.
As an example, we're starting at 8 AM and sunset is 7 PM. We have 11 hours to hike in the light. Dividing by 2 we have 5.5 hours to get to our furthest or halfway point. That means at 1:30 PM we turn around and head out.
The most important thing about turnaround times is to follow them!
Now, as you get more experience with hiking, and get to know how you and your group hikes, you can change things up based on the terrain. For me personally, I know what my paces are on various slopes and terrains. So I will calculate time markers for myself to set the times. This is a much more complicated method, and it will fail if you don't know your typical pace or the groups typical pace. But, assuming you do, I'll go through how I calculate things.
Advanced Example
Lets do a hike up Algonquin from ADK Loj. This is 3.6 miles and 3000 feet of elevation one way. The way I'd describe the trail is the first mile is ADK "flat". The next half mile starts getting steeper and rockier/muddier, but isn't horrible. At about 1.5 miles the climb starts. We have about a mile of steep (20-30% ish) rocky trail, then about 3/4 mile of steep slabs, and the last .3 miles is all steep scrambles.
How do I hike going up and going down these terrains?
ADK Flat is about 2 MPH either way. Under 20%, typical trails I'll average about 1.5 up and 2 down. Steep 20-30% trails I'll be about 1 up and 2 down. The slabs, I'll be about 1 up and 1 down. The scrambles at the top I'll be more like 0.5 up and 1 down.
Now we do the math:
going in:
1 mi x 2 mph + 0.5 mi x 1.5 mph + 1 mi x 1 mph + 0.75 mi x 1 mph + 0.3 mi x 0.5 mph = 0.5 h + 0.75 h + 1 h + 0.75 h + 0.6 h = 3.6 h hours to reach the top.
Coming out:
0.3 mi x 2 mph + 0.75 mi x 1 mph + 1 mi x 2 mph + 0.5 mi x 2 mph + 1 mi x 2 mph = 0.3 h + 0.75 h + 0.5 h + 0.25 h + 0.5 h = 2.3 hours to come out.
So, in total, I expect to take 5.9 hours to do the hike, lets round it to 6 to make life easier. That's an average pace of 7.2 mi / 6 h = 1.2 mph. That is actually slower than my typical average pace of about 1.5 mph (which comes out to about 4.8 hours), so I know I've used some conservative estimates and I feel better about this estimate. That means that if I was to do this on the winter solstice when the sunrise is 7:28 and sunset is 4:19, just under 9 hours of light, I can hike the whole hike in daylight. I also know that I expect it will take 2.3 hours for me to come out. Lets round that up to 3 (always be more conservative in your estimates here) and my turn around time to not be hiking in the dark is 1:19, or 3 hours before dark. If I'm not heading out by then, I'll likely be hiking out in the dark.
The most important thing about turnaround times is to follow them!
Yes, I am repeating myself because it is a very important concept. There have been a few times that I didn't turn around when I hit that time and I paid the price. The example I had in the trip planning article I followed the turn around time and my hiking partners didn't with them coming out at 1 AM instead of a reasonable hour. When I went back to get Hough through a different route, I didn't follow my turnaround time and I was about an hour late when I finally hit the peak. Instead of coming out around 7 when I planned, it was closer to 9, and both myself and my hiking partner bonked on the way out. Myself about 4 miles from the car where I needed protein so I ate my emergency can of tuna. My partner about 2 miles from the car where she needed more sugar/carbohydrates and a couple cookies helped her. It was a long day with 15 hours of hiking in the middle of the winter.
I have to stress once more, you need to follow your turnaround times in order to have an enjoyable and positive experience in the woods. Not obeying it will lead to lessons learned and experiences you didn't plan or wanted to have.