weapon spells

do these only last for a single hit or do they stay for the stated spell duration?

different ones have different effects.
splinter weapon: 1 hit
brutal weapon: ~30 seconds (or somthing like that)
just depends on the spell

Kinda like Protection spells. Some only last one hit, others last for awhile.

Just remember though, that Weapons Spells are mutually exclusive (ie they don't stack)