Originally Posted by
Aviatrix
Things may have changed (I haven't used lockers at German train stations for a few years) - but it used to be the case that lockers came with a maximum time period of something like five days. Anything not retrieved after five days would be treated as lost luggage.
At the FRA airport, the luggage storage is a staffed office, not lockers. There is a daily charge, but a week should be fine (verify of course when dropping off). However, the bigger problem is the short 2 hour timeframe between flights.