The volume of the tank on your E-class is 8,5L or 24,5L.
If the level of AdBlue® reaches approximately 3,0L, the message "Add AdBlue®, see instruction manual" can be read on the dashboard. Following the fill-up, if you drive faster than at 10 miles/hour, the message "Add AdBlue®, see instruction manual" disappears after a minute. When the AdBlue® level reaches its minimum level, the message "Fill up Adblue® autonomy: ... km" appears on the dashboard. In a situation in which the mileage appears, you can still drive for the indicated autonomy.
When the signal goes of, you have to fill up with AdBlue® as soon as possible to reactivate the engine. If you don't fill up with AdBlue®, you will not be able to restart your vehicle. The message "Add AdBlue®, start impossible" appears to require a fill up. When the tank is empty, pour at least 3,5L to switch the warning light off, switch on the ignition for at least 60 seconds and then start the engine.
This information can vary on some vehicles of the same model in relation to the options chosen by the manufacturer.