How deep should we discharge lithium batteries to maximise their lifetime?
My quick look into it:

The lifetime of lithium batteries decreases with the depth of discharge, looking like the following (this curve is for lead-acid batteries, but Lithium is stated as following a similar curve):


(source)

If the 100% DoD value is taken as a reference, one can plot what I call the "isoenergy" curve (I gave it a 2sec thought) which is basically how many cycles are required from the battery to deliver the same amount of energy as 100% discharges over its entire lifetime: $$isoenergy(DoD)=\frac{100\%DoDlifetimecycles}{DoD}$$
For example, 50% DoD require twice as many cycles as 100% DoD, 25% four times etc.

The results with this particular example:



Conclusion, it still holds that the depth of discharge should be minimised as much as possible.