Spell Tinkerer (Su): The arcanist can alter an existing spell effect by expending 1 point from her arcane reservoir. To use this ability, she must be adjacent to the spell effect (or the effect's target) and be aware of the effect. She can choose to increase or decrease the remaining duration of the spell by 50%. This ability can be used on unwilling targets, but the arcanist must succeed at a melee touch attack, and the target may attempt a Will saving throw to negate the effect. This ability cannot be used on a given spell effect more than once. This ability has no effect on spells that are instantaneous or have a duration of permanent.
Mage Armor modified by Spell Tinkerer = 1.5 hours per level
Benefit: An extended spell lasts twice as long as normal. A spell with a duration of concentration, instantaneous, or permanent is not affected by this feat.
Level Increase: +1 (an extended spell uses up a spell slot one level higher than the spell's actual level.)
Mage Armor modified by Extend Spell = 2 hours per level
Would both of them used together follow the multiplier rule of adding multiples together, for 2.5 hours per level instead of 3?
Extended spell would make the duration 2 hours per level, as that is a "base" duration once the metamagic spell is memorized essentially. Let's say it was 4th level caster..
Your duration would be 8 hours.
After that you would apply the exploit to increase the now ticking duration of 8 hours once it cast. This would mean either increasing the duration by 50% (plus 4 hours to 12 hours) or decrease it by 50% (minus 4 hours to 4 hours).
The exploit is figured after the spell is cast, so doesn't come into play in the way you are asking.