When will my per diem be taxed, what percent is it taxed, and how much will I get back filing taxes?

My employer is sending me out of state for 12 months (fulfilling a 12 month contract). Since I will only have to pay for food (lodging covered), we agreed to a specific number. However, I learned that I could be taxed since the contract is for a year. At what point will by per diem be taxed (starting day 1 or anything over 365)? What is the federal tax percent they tax per diem and how much will I get back when I file? This is not a new contract for the company, im just the replacement. Meaning, I am replacing someone at the location for 12 months where someone else will replace me.