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That looks like a decent candidate for repair with some wood spliced in with an epoxy resin like repaircare. Complete cill replacement would be more time consuming and needs some interior decoration.
If it's just that work then it would need to happen over at least 2 days. I don't think it would take 16hrs but even with the best conditions it needs time to dry between coats of paint.
If you've no other work going on that's a lot of standing about.
For that reason it's not unreasonable. You also need a lot of small quantities of different materials which is a bitch to price.
Been quoted £576 to repair/replace this sill. Seems like a lot…