I hope this goes to the right place!!
It varies with region and circumstances. There are several factors. Soil can get buried, washed away, gullied, and sometimes worst, saturated with salt.
Gullied or buried agricultural land might be salvageable (expensively) with bulldozers and the like. If that is a practical option (IF!) then you could be up and running in a season or so. Bedrock could be covered with topsoil (if any! Topsoil is expensive stuff in high demand) within a season or so. Alternatively, if you have no soil, you could build up new topsoil from scratch from subsoil and bedrock (it literally could take dynamite!) by proper tilling, green manuring, choice of crop, and so on, and you even might thereby release minerals that were in shoer supply before, but it is a slow, contingent, and expensive process. It could take several years.
Salt can be a blighter. if the soil is not very penetrable, so that the water flows over and goes away, it might not be too bad (some hope!). If there is well drained soil with lots of rain things could settle down in a few years, but with bad drainage and lots of clay, you could kiss that land goodbye for generations. Where land is at a premium intelligent choice of crops could extract some of the salt and speed up the process, and with real intelligence we could apply GMO crops to do it fast and profitably.
