Following the questions on FB , here is the definitive answer from the Fiat Proffessional page..

When do you need to top up the AdBlue® tank?

AdBlue® consumption depends on your driving style. The instrument panel will alert you by means of a warning light, a symbol or a message when your car needs an AdBlue® refill.
We advise you to top up with AdBlue® regularly, especially before long journeys.
If the AdBlue® tank is empty, you will be unable to start the engine!

How do you fill up with AdBlue® ?

You can fill up yourself following this simple procedure, or contact our Service Network.
Always remember to refer to the full instructions provided in your Owner Handbook

1. LOOK OUT FOR THE ADBLUE® ALERT MESSAGE OR WARNING LIGHT

If the AdBlue® level is low, the warning light or symbol illuminates and a message advises you to top up with at least 5 litres of AdBlue®.

adblue
2. FIND THE CAP OF THE ADBLUE® TANK

The cap is next to the fuel filler cap, but in some models it could be in the engine compartment.

adblue
3. FILL THE ADBLUE® TANK

You can fill up using a filling pump, can or bottle. Remember always to fill the tank with at least 5 litres of AdBlue® If AdBlue® spills out of the filler neck, clean the area thoroughly and continue filling; if the liquid crystallises, remove it with a sponge and warm water.

4. WAIT FOR THE MESSAGE TO DISAPPEAR OR THE ADBLUE® WARNING LIGHT TO SWITCH OFF

After filling up, with the ignition device in the MAR/RUN/ON position, wait for the warnings on the display to go out before starting the engine. If the reservoir was empty before filling up with AdBlue®, you may need to wait for up to 2 minutes.

Storing AdBlue®

AdBlue® is considered a very stable product with a long shelf life. If is stored at temperatures between -12 ° and 32 ° C, it can be stored at least for a year.
Follow the instructions on the label of the container.

AdBlue® freezes at temperatures lower than -11 °C.
Close the AdBlue® container firmly and store it in a cool, dry, well ventilated place. Do not expose the container to direct sunlight.
How do I find out how much Ad Blue I have in the tank? 
If you go through the menu on your indicator arm it should tell you the level of adblue. Being a fleet manager I can give you a little bit of advice. If you’re adblue warning light comes on, you will have around 40?miles before you need to fill. If the warning comes on and you stop at a garage Don’t stop the engine while you fill, because if you do it won’t start, and if you run too low your vehicle will go into limp mode but that will give you chance to get it to a garage. 👍🏻