Travelling to my father in Spain tomorrow. It will be the first time for me where free roaming is available. With my operator I get 7 GB data to use as I wish while in another EU country.
How demanding is Hearthstone? I don't want to waste half a gig just playing one game of Hearthstone, but from experience there will be many evenings where I'm fucking bored, so some Hearthstone would lighten those evenings up a bit.
You could check your own Hearthstone data usage for the past 30 days if your using windows by going to settings > network and internet > data usage > view usage details > hearthstone. Pretty sure both ios and android has this feature aswell.