1 Minute per paladin level (per use of Divine Bond)
(Divine bond, page 32)
"When called, the spirit enhances the weapon, causing it to shed light as a torch for 1 minute per paldin level. At 5th level, this spirit grants the weapon a +1 bonus. For every..."
I'd say the spirit will stay around for one minute/level. At first, it will only illuminate things, but later, it will give it the bonus, all for the duration mentioned.