Before you buy the World of Warcraft 30-day prepaid card you should know...
WoW Time Cards have a regional block, which means that activation must be made from specific regions. For example, this 30-day card for WOW is for the United States only, so we must be in that region to activate it.
If you are looking for the 30 days WOW Prepaid Card for Europe you can access the list with the best prices from here
This world of warcraft prepaid card will offer us 30 days of continuous play, but when it's over, you'll need to buy it back or activate automatic subscription from your Blizzard account.
Buying prepaid cards individually is much cheaper than maintaining the subscription automatically, as Blizzard will charge you 12.99€ per month from their store, but if you buy the card as a code you will have the same playing time in WoW for a lower price.
To activate this type of time card, simply enter in your Battle.net account the password that you will receive in your email after purchase.