I'm based in London, so dropped it off with https://www.ismash.com/ in St Pancras. Was done in half an hour and cost £40. Can't vouch for the quality of the battery yet, but they are a pretty big chain now so I'm assuming it won't be awful.
However I've had batteries replaced (different phones) at the Apple Store before and it took about an hour.
However I've had batteries replaced (different phones) at the Apple Store before and it took about an hour.