My 2016 Sprinter Operators manual says the capacity is approximately 4.7 gallons of DEF.
My practice for the last 50,000 miles has been to check the gauge at random intervals and when it is just below half (just as the OPs photo shows) I stop at Walmart and buy a 2.5 gallon jug and add the whole thing. No muss, no fuss and no storing half full jugs of nasty stuff. Occasionally it does take a while for the gauge to reset. I have never waited for a warning light to add, I do it proactively.
I’m currently on my first trip since having the warranty recall work performed and it seems that the DEF is not being used as fast as in the past, but I have no evidence…
The Sprinter maintenance manual, on page 8, Checking and adding DEF, says ”Check and refill DEF every 20,000 miles. Don’t think that would work very well…