Radiation is the type of contamination that takes the longer to clean up.
The lowest level of radiation takes 34 years to clean itself naturally. Higher levels are multiples of that time length.
A year is 12 months or, for the player, 12 days. Which means that a whole level of radiation goes down every 408 days. Indeed, it is a long time. We are talking about radiation anyways.
Trees and (in a fraction of effectiveness) Parks help clean contamination slightly faster, just like for Ground and Air pollution. However the process will be considerably slower, and those the only tools you got to deal with it.
For more information about dealing with pollution, check out the Pollution guide.