60 to 90 minutes on land per landing. Each excursion slot runs about 2.5 to 3 hours off the ship in total, split between time ashore and time in a Zodiac. That sounds short until you're standing in it. Most people are ready to head back after an hour and a half, and the ones who want longer are usually serious photographers.
This is the number that should defuse the small-ship anxiety. People choose vessels to maximise shore time and then discover the constraint was never the ship, it was how long you want to stand in the wind next to a penguin colony.
You're not walking far. Guides scout and mark a route, and crevasses and cliffs are kept off limits, so the area you can cover is deliberately bounded.
The other half of the slot, the Zodiac cruise, is where a lot of the best moments happen. Whales, calving glaciers and ice you can't reach on foot.