It depends on the exact model. Bobby pins or any key that physically fits will get you into some, you'll need real picks for others, and some custom jobs may require a police snap gun or a locksmith's services. This doesn't correlate with aircraft price: the larger and more advanced, the less likely it is to have any locks at all. At the high end, jumbo jets have no locks and have a (not so) convenient hatch in the gear well, so you can climb inside even without a ladder.
Mechanical locks, with the exception of the more robust types used on burglary-resistant safes and vaults, offer only very superficial security. Airplanes are protected against professional threats by external access restrictions (fences, hangars, guards). When parked at a small local airport, opportunistic theft is deterred by "security through obscurity": very few people are able to operate one, with model-specific skills are involved, and even fewer could make use of a stolen jet. Unlike car parts, aircraft parts come with paperwork.
The locks used on most aircraft doors are small pin and tumbler cylinder types that can only protect against passers-by. The stock locks on the most popular light planes even use wafer tumblers, which offer no resistance to picking - some light bumping with a key from another lock of the same type will get it open. Quite a few owners have opened plane doors with a wrong key by accident.
If you're not concerned about property damage, as in an emergency situation, no picking skill is required to open the kind of locks you'd find on a private aircraft. A non-matching key or a flat-head screwdriver, jammed into the keyhole and forcefully rotated with a pair of pliers, can break small cylinder locks' pins and allow the lock to open. Larger locks require a bit more effort, but it's all fairly intuitive work of destroying what's in the way of opening the latch.
In non-pressurized aircraft, you can also bypass many locking mechanisms by applying well-directed force, which slightly deforms the door to allow the latch can exit the door frame. In some cases this can be done non-destructively, just by elastic deformation. Pressurized doors fit too tightly for this, but the lock is still a standalone device, not tied into the door's operating and sealing mechanism, so damaging it won't leave the plane inoperable.
24 hours is far more time than would ever be needed. 1 hour is enough time to get a professional locksmith, which, with proof of ownership, can get you into any stock aircraft/boat/car/other vehicle with usually no damage to the lock. Another hour will get you a new set of keys, or a change of locks with new working keys.
The time it takes to get into a locked aircraft, in an emergency situation, should be between 20 seconds and 20 minutes, depending on the plane, the characters' skills and equipment, and their willingness to damage the lock. For a larger jet you'll definitely be going after the lock, not the door.
The only entry-resistant doors in aviation are those separating airliner cockpits from the passenger cabin, with electronic locks and solid locking mechanisms. They're rated against different threat models than UL vault doors, but my best estimate based on their construction and weight would place these doors at around TL-15 level, with about 15 minutes of power tools resistance.
For reference, the highest ratings for safes and vaults are TRTL-120 and TXTL-60, indicating 120 minutes contact time resistance against power tools and torches, or 60 minutes against explosives and small thermal lances. In absence of a security response, 24 hours and a good hardware store can get you into Fort Knox.