On average, chicken litter will contain 50 lbs of N, 50 lbs P, & 50 lbs K per ton. This will vary depending upon moisture and if the litter was from a total clean out or a de-cake.
Availability of the nutrients will vary. As a rule of thumb, I will credit 25 lbs N, 40 lbs P. & 50 lbs K per ton of litter.
You will get the best use out of litter by soil testing your fields and identifying those lowest in nutrients. If your P & K soil test levels are already high, it may be cheaper to just apply N fertilizer to your fields.
In situations where soil P & K are low, 2-4 tons should meet the crops nutrients needs in addition to providing removal rates.