How long does it take salmon to swim upstream to spawn?

It can be a few days to several weeks, depending on many factors...the health and stamina of the fish, the conditions of the river, how fast the water is flowing, and how far up river the fish needs to go to return to the point or near the point where it hatched. The salmon must reach a point far enough up the river where salt water can't reach the eggs. Some salmon, for example the humpback or pink salmon that spawn in the Indian River in Sitka, AK, the trip can take just a few days. For others, for example, the Chinook, or King salmon that travel up the Feather River in Oroville, CA from the San Francisco Bay, it can take weeks.