This is the structural decision point. With 6–8 hours, a city trip becomes structurally possible — but not for everyone. Your baggage, booking type, Schengen status, and terminal determine whether leaving the airport is reasonable or whether you should stay inside.
Of your 6–8 hours, you need: ~30 min to deplane and orient, ~30 min each way for S-Bahn to/from city centre (including walking and waiting), ~90 min before departure for security and boarding. Realistic city time: 2–3 hours. If any factor below adds time, your city window shrinks.
Through-checked or cabin only: no baggage overhead — full city time available. Must collect: baggage claim + re-check consumes 45–90 minutes. This alone can make a city trip impractical. Check your baggage tag destination code.
No border crossing: no passport control — maximum flexibility. Crossing Schengen border: passport control required both ways. Add 20–45+ minutes depending on queues. FrankfurtFinder does not estimate queue times.
Protected booking: airline responsibility for rebooking if delayed — lower risk. Separate tickets: you bear the full consequence of missing your connection. A city trip adds risk that you alone absorb. This does not change the structural time budget, but it changes the consequence of delay.