How long do wild snakes typically bask in sunlight/heat?

How would anyone know the average basking time of a wild snake? Unless they sat there and watched them for days?

Depends on the air temperature. If it is a cold day, they may bask longer. If it is a hot day, they may not even come out until the night time. Many snakes do not even expose themselves to the sun, but simply hide under a rock or a piece of sheet metal and let the rock or sheet metal heat them up.  Many snakes are killed while they press themselves on a concrete road to absorb the warmth. I have also seen snakes stay inside a hole in the ground and expose just a small part of their body to the sun through the opening.

Many snakes prefer to move around during the day, when they can alternately bask in the sun's heat and slither into shadows to regulate their body temperatures. ... To maintain body temperature, snakes need heat, not necessarily sunlight -- but sunlight is a generally the most convenient source of heat for snakes. Each species is different. As is each snake. But generally just until they warm up enough to move.