It takes me about 40 minutes to pack for an overnight or longer trip, and this includes making and packing sandwiches for breakfast and lunch.
The biggest advantage is that I know exactly what I need, where it is, and how to place it in the pack. This is mainly the result of going on hikes nearly every weekend year-round, and learning what I use and don't use, and what I find that I'm missing in certain situations. A lot of gear never leaves the pack unless it needs replenishing or for routine maintenance (I check and refill all of my different kits about once a month, unless I use them. This also allows me to adjust them for a more localized climate window.)
Biggest factors in my packing time:
- piling -- I pile everything I'm going to take on the floor in logical groups, next to my pack. This allows me to assess whether it's too much, what size pack I need to use, and make sure that I have everything I need.
- clothing -- knowing exactly what I'm taking and where it is helps reduce the time. I generally have a separate wardrobe for technical hiking gear, which allows me to keep it separate from the rest of my clothes.
- food and cooking gear -- again, it helps to have in one location, know exactly what you're taking, and have it all pre-filled (stove fuel, food, spices, etc.) I try to pack any cooking ingredients for the trip the night before or earlier. Simpler recipes are easier to pack (e.g. rice, a bit of oil, nuts, and spices take all of one minute to pack, premixed in a reusable ziplock (except for the oil, of course)).
- overnight gear -- again, one storage location, one packing pile on the floor. This includes the sleeping bag, sleeping pad, compression sack, and possibly a tent.
- kits -- this includes various kits, like first aid kit, backcountry ski binding repair kit, stove maintenance kit, backpack maintenance kit, emergency shelter kit, etc. I keep these as "kit-in-a-bag" systems, so they're always ready, tabulated, and packed. Some of these stay in the backapck between trips.
And finally, to emphasize the point: the biggest time saver is knowing what you don't need. The less you have to pack, the better it fits, and the faster it is to collect and review it. The second biggest time saver is knowing how to pack what you're taking in the pack you're going to use, so maybe spend some time packing it with different sets of things so that when you're packing for a trip you already know exactly how to pack the stuff you're taking, as well as that it fits and that it's not too heavy.
Occasionally I will actually try different packing orders in my packs on a dead evening, just because it helps me later.