Are you asking about salt as in table salt?
If so, how much was added to (or is in) the soil?
And why was it added?

The time til it's gone varies on what happens next.

If the salt is the sodium of table salt, mix in gypsum and water thoroughly to flush the stuff out.

Depends on where in the United States you are and the amount of rainfall you get, or the amount of water you pour on the soil, as well as the amount and type of salt. Gypsum is known to help move the sodium (salts) out of soil a bit faster than just water.