How long would it take for radiation levels to go down after a nuclear war?

go down to what?

radiation decreases exponentially so can NEVER be zero

the half life aint when something is safe- its just when the radiation is 1/2 what it was

i.e it takes 2x as long to get a lethal dose

the nuclear winter aint so much the radiation as the dust the bombs blow into the atmosphere - This would block out the sun and could result in a global ice age (i.e everything dies)
the dust could persist for decades (depends how high it goes)

Seeds might survive in the ground then regrow (depends how far down they are)

WIth 5000+ megatons - there is no question what would happen

1000 meg- probably would happen

100 meg - unlikely

Depends on what radioactive materials come about as a result of the war. Some have a half life of centuries. If a nation deliberately uses dirty bombs it could be that the radiation level never goes down enough and we all die. As far as nuclear winter, figure 2-3 years. If you do not have enough food to get you thru, you die. If you do not have the means to plant and grow new food after the winter is over, you die. You pretty much need to be in an area that does not get nuked, does not get fallout and can grow food. Plus even if you have food and can grow food, there are going to be those who want to come along and take what you have.

1-5 years.