If it's anything like other systems, like Range Rovers for example, you'll get multiple warnings until the eventual "no engine restart" to which I'm pretty sure you better have some DEF on hand because it really won't restart.
Now it's just a matter of how long can you go before it gets empty and leaves you stranded. I guess once you keep driving.. you should be alright?
With the DEF filler cap under the fuel door next to the green fuel cap, I'm not expecting any problems after the extended service that I paid for in advance is all used up... I will still be taking my rig to the dealer until the 100K extended warranty is used up. Having twin turbos and piezoelectric injectors, I did want the added security but now I'm spoiled and so far it has been been easy, problem-free driving for us (knock on wood). The miles between fuel stops is great and not having any problems with the DEF tank going dry between service intervals is part of the package: e.g., >30 mpg on a 5,000 lb. SUV with exhaust that passes the white handkerchief test.
Do you know what the levels are right before you go for your next service? As in, is there a lot to spare kind of deal?
The 2015 ML (GLE) AdBlue tank is 7 gallons and, apparently a lot to spare...
We've got a 2015 "250" with 47005 miles and prepaid service and it does get better mpg than our previous 35OBTC so it probably uses a bit less AdBlue every 10K miles.
The 40k service invoice says, "1/2 GAL OF ADD BLUE." Ditto for the 20.5K service. The 10.4K service says, "//CORRECTED ADBLUE LEVEL. 4 GLS"
So, I'm thinking MB's little diesel uses ~1/2 gallon of AdBlue every ~10K miles and the 7-gallon AdBlue tank is only filled with 3-4 gallons to begin with (or, perhaps 0.4 gallons were added at the 10K service).