Incheon -> Beijing -> Copenhagen -> Iceland

I'm Korean and my transit route is Incheon -> Beijing -> Copenhagen -> Iceland.
Transit time in Beijing is about 5hr, and I'm not going to go out of airport.
Should I get 72 hr transit? or 24 hr transit?

Is it different by below cases?
1. get boarding pass only from Incheon to Beijing
2. get boarding pass from Incheon to Copenhagen. (partial)
3. get boarding pass from Incheon to Iceland. (entire route)

Thank you.