How long does it take to replace the battery in a MacBook Pro 15inch 2018 model

How long depends on your model, parts availability, and where you live. We are not Apple so cannot quote repair times.

The is about a 90+% chance that you will not be able to get the work done in an Apple retail store while you wait. Typically they will send the computer to one of Apple's centralized refurbishment centers where a refurbished deck fitted with a new battery, keyboard, and trackpad will replace you existing parts, all for a fixed cost that is about a third of what and independent shop would have to charge you.

The same service happens if you use Apple's mail-in/ship-in service.

The advantage over taking it to an Apple store is that you save about a day on each end because the Apple store is going to send it the the same place anyway. The servie deliver it to your home or designated location.

If you are not in N. America, the ship-in option may not be available. Start here to see pricing and for which service options you quailfy:

Mac Repair & Service - Apple Support

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+1 on @VikingOSX's recommendation.

Replacing the battery in a 2018 MBP is a very involved, difficult procedure. It is not DIY. Make an appointment at an Apple Genius Bar or take it to an Authorized Apple Service Center.

Confer with an Apple Store, or Apple Certified Service Center. That will provide you with the assessment of the current condition, replacement time and expense.

As these are fellow user supported communities, Apple is simply not here to answer your question and best to ask those that would be directly involved per the first paragraph.