My friends and I are about to start a new Pathfinder campaign, and for various reasons we probably won't have many/any beasts of burden to start off. However, most of the people in my group are still pack-rats and want to carry enough equipment that all players will be under medium or heavy load. None of us want to deal with the consequences of fighting with the restrictions for carrying more than a light load. We imagine most of our gear would be carried in large rucksacks or backpacks while traveling and I was wondering if there was any precedence or rules on how long it takes to shed gear (primarily for combat reasons).
It depends on what they drop, and how it was stored.
Held item: These are whatever objects the character has in their hand(s). Dropping or releasing an item from your hand is a free action.
For the other types of items, a character will need to first unequip or retrieve the item, before they are able to drop it as a free action. This means the process to drop the item may take longer.
Stored item: Most items are stored in locations like belt pouches, backpacks, pockets, and so on. The rules for actions in combat state that retrieving a stored item is a move action, although certain items may specify otherwise (Example: searching for a specific item in an over-stuffed Bag of Holding is a full-round action). Retrieving an item usually provokes attacks of opportunity, so in practice, a character in a threatened area may want to move first, before retrieving the item.
Sheathed weapon: Sheathed weapons are stored items, so they require a move action to retrieve or draw. Characters with at least +1 BAB can draw their weapon as a free action as part of their movement. Characters with the Quick Draw feat can always draw a weapon as a free action.
Shield: These are typically strapped to an arm, and so they normally take a move action to unequip. However, a character with at least +1 BAB can move and drop their shield as a free action, as part of their movement.
Plate armor: Equipping and unequipping (half or full) plate armor is a complicated process and is probably too slow for combat. This normally takes 1d4+1 minutes, although this time can be halved if someone assists the character to remove the armor.
Other armor: If it's not half or full plate, then it takes 1 minute to remove. This time can also be halved if they receive assistance, but this may be very difficult during combat.
Other worn items: This includes clothing, accessories, slotted Wondrous Items, backpacks, pouches, and so on. Because these articles vary so much, it's possible that the unequip time varies as well. However, manipulating an item is a move action. If removing a worn item counts as "manipulating" it, then perhaps the default (to unequip/drop the item) is a move action.
Side note - If you're concerned about encumbrance penalties, have you considered ways to improve the character's carrying capacity? Certain items like Muleback Cords and Heavyload Belt may help becoming over-encumbered.
In the Actions in Combat section of the SRD, we see that "drop an item" is a free action, but that's intended for something you just are carrying in a hand - like a weapon or torch, or a bag. There's not a listed action for shedding e.g. a backpack but I think it's fair to say it's similar to 'drop a shield', which is a move action.
There are a bunch of other described action costs for rummaging through packs etc., I assume you're aware of those and just asking about the missing part which is the cost of shedding a backpack or belt pouch or other container item.
P.S. I am sure you'll get some answers saying "don't do this it's badwrongfun" but a game style where you sweat the equipment details is completely legit. Just make sure you care about the whole encumbrance/ammo/etc thing, you don't have to if you don't want to.