It really does depend on the complexity of the damage. the size, and materials that are being used but average size window sill plate is a fairly simple project should require no more than A few hours to comple.
It will depend on if the window or casing also needs to be removed. But assuming you just need the sill plate repaired it will take 2 hours. The price of $1K is reasonable if you are hiring a general contractor and this is the only thing you have them doing (lots of overhead, insurance, etc) and about 4x what it would be if it were a handyperson. If you have other work that can be done by the GC I would project the cost would be half.