How long can a level 30 commander hold a captured officer?

it isn't actually based on the level of the commander but by the level of that pertaining commander's "Prison". It may be the case that the commander is level 30, but his/her prison is just around level 1 or so. Nonetheless, a level 30 Prison (which requires a level 30 HQ and a ton of Thick Wire [item bough in the store]) would hold an officer captive for a total of 1 day and 6 hours. Take note that you would also need a "Super Glue" to revive that officer.

My officer has been held for a week without being executed.