Enough that if you are going to do it, replace the belt at the same time.
And depending on its age, the water pump and thermostat and all your belts since you are there. Check your seals. I did this job myself using the factory manual. You have to remove the radiator first, which gives you enough room to work. The hardest part may be loosening the crank bolt, but others have posted that you can use the starter to help you. The only other thing that made me crazy was that after you line up the three timing marks and turn the crank, the marks on the belt move a tooth, but the marks on the crank pully and each cam pulley still line up. Ignore the belt markings.Are we talking 3.0?
It's a PITA. This engine is like an onion; you have to peel the layers off to get to what's underneath.
That said, you probably need to replace BOTH idler pulleys & and the belt.