It works out at 120 days since the last topup. However if you maintain a positive balance the card stays valid.
Have a look at:
blau - Preguntas frecuentes
The problem is that the 2 euro/month minimum consumption can bring a positive balance to zero if the SIM is not being used. After the card hits zero, you can only receive incoming calls for 30 days, after which you only have 90 days more in which to topup or the card becomes completely disactivated.
Remember that this month, if you put 30 euros on, you will also get a bonus 15 euros added on next month!