Sorry.
Unlimited use, always on. Main problem I'm seeing is the spell is too darn short duration, so you have to effectively suspect a trap before you cast it.
A continuous effect has a cost of spell level * caster level * 2000. For a 2nd level spell (minimum caster level 3) that puts the cost at 12000 GP. It'll cost an extra 4000 GP for each additional caster level you want to increase the effect by.