In my case I have a Celestial Armor (CL5) and a Celestial Shield (CL7) The armor lets me fly once per day (as the spell) and the shield gifes me feather fall once per day and because I have both I could use overland flight rather than fly. Do I use my caster level to determine the duration of those spells or the CL of the magic-items? fly=5 min., feather fall=7 rounds, overland flight=5 hours or do I use my own or the highest of the two?
For determining duration, as well as any other CL dependent variables such as DC for dispel magic, you'll use the Caster Level of the item as laid out in the stat block.
Caster Level (CL): An item's caster level indicates its relative power. It also determines the item's saving throw bonus, as well as range or other level-dependent aspects of the item's powers (if variable). It also determines the level that must be contended with should the item come under the effect of a dispel magic spell or similar situation.
From the PRD Magic Item page
So, for your specific cases: Celestial Armor Fly: CL 5 for 5 Minutes 1/day Celestial Shield Feather Fall: CL 7 for Until landing or 7 rounds 1/day Celestial Armor Overland Flight: Cl 5 for 5 hours 1/day