I built 5" quad and I'm using 4S 1300mAh LiPo batteries. My question is: at what voltage should I land? Let assume that I'm trying to land when my battery is as low as it should go without causing damage to the pack.
I start my landing approach when the average cell voltage reaches below 3.5V on a stable flight (not counting voltage dips on short, aggressive maneuvers).
At that point, you should still have enough authority to land safely. You can go somewhat lower without damaging the cells, but at that point, the voltage curve starts to be very steep and it is increasingly easy to slip below dangerous levels.
I wouldn't advise landing on 3.2V simply because it's too easy to overshoot and to me, it doesn't give meaningful airtime over landing at 3.5V.
Also, note that there can be a low voltage cutoff set up in the ESC that you probably don't want to trigger while in the air.
I have found that landing based on voltage is inaccurate. For example, if you look at the voltage in the OSD while you have the stick at full throttle, the Voltage will be lower than if you had it at half throttle. So, do you land any time full throttle goes down to 3.2V/cell, or when you are at half throttle and 3.2V/cell? Instead of relying on voltage as the indicator, I've gone to looking at mAh usage (Current in the OSD). Note that if you choose to use the current sensor, it must be calibrated because there is no guarantee that it will come calibrated from the factory. An current sensor that is not calibrated is as effective as landing based on the amount of time you are in the air.
With a 1300mAh LiPo, you can use up to 1040 mAh (80% capacity of the LiPo used, where 1040 is the critical point at which your LiPo will likely start to see damage to some degree). I tend to set warnings at 910mAh used (70% capacity of the LiPo), and then try to land by 975mAh used (75% capacity of the LiPo). This puts my batteries at, or near, storage charge. Using mAh (current), instead of voltage, makes it easier to gauge when you should land.
Here is a chart I made that I keep in my drone bag as a reference, so I don't have to do the math every time I use batteries of different capacities.
My personal strategy is to use both voltage and mah as guides. For a 1300mah battery, I'll fly around and if I notice I've used up 1000mah I'll come in and land. However, if I'm flying around and I notice my voltage osd warning blink is happening more and more often during even nominal throttle use I'll come in and land. In the latter case, the battery is surely old and has a diminished capacity.