Vastauksen tähän kysymykseen jakoi irritus
If you top up your Saunalahti Prepaid, it will be valid 12 months from the last time it was topped up.
The only definitive way to find out if your sim is still active is to put in in a phone and try out if it works. Obviously, in order to do this, you have to be either in Finland or one of the countries where Saunalahti Prepaid roams.
https://lataa.elisa.fi/ can be used for very rudimentary check. Enter your phone number. If it tells you The subscription number entered is incorrect. Please check it and enter again, your prepaid has expired. In that case there is nothing that can be done, you must get another prepaid sim.
If you do not get the forementioned error right below the phone number, then the number is valid and can be topped up. However, in the worst case, if you didn't top your prepaid up and the number expired in November 2016, it might have already been reassigned for someone else.