Calculating spell durations:

Self-cast spells 
If your character knows 35 ranks of Minor Spirit, when you cast Lesser Shroud (spell 120) the duration will be 1200 sec (Base duration) + (60 sec x 35) = 3300 seconds, or 55 minutes.

For items invoked from scrolls or activated from magic items, durations are dependent on the user's Scroll Reading and Magic Item Use skills, respectively. Every two ranks are treated as knowing one spell of the applicable spell circle.

If your character has 140 skill (40 ranks) of Magic Item Use, activating Spirit Warding II (spell 107) from an imbeddable item would give your character: 1200 sec (Base duration) + (60 sec x (40/2)) = 2400 seconds duration, or 40 minutes worth.

NOTE:
Drinkable and edible magical items are the only exceptions to this duration. Drinkable and edible magical items will provide only the base duration with no additional bonus for Magic Item Use.