Fonic SIM cards never expire. Officially, however, your credit will be blocked 6 months after the last recharge. If that happened, you can still receive calls for another 6 months, but you can't call out due to the lack of credit. But then you can still release the blocked credit or re-activate the SIM (if 12 months have passed) by adding additional credit.
I think validity is shown somewhere on their website, but you can also try entering *102# [pick up] on your phone, which at least will display validity for other O2-based SIM cards.
Online top ups require a German bank account. You will need to buy top up vouchers from local retailers (see
Germany - Fonic - Prepaid Wireless Internet Access for a list).