How long does it take to get a 6-month multiple entry visa for Thailand?

Honestly it's one of the most difficult visas to time when to apply.

If you're trying to maximize your time in Thailand, you want the visa to be issued as close to when you'll travel as possible. Because the 6 months of the visa starts from the day it is issued.

How long it takes will depend upon what embassy you are applying through and if there are any issues with your application.

London issues visas in 2-3 days often, whereas Paris and Ottawa can take months.

A normal timeline is 15 days but that's no guarantee. You're probably going to want at least 3 weeks to give a little leeway.

Make sure for your application you send them justification why you need "multiple" entries. This means a ticket into Thailand, a ticket out of Thailand within 60 days, and then another ticket back into Thailand. Without this you run the risk of them issueing you a single entry tourist visa since you didn't justify your need for multiple entries.

Depends on the consulate responsible for you. But might take up to a couple of weeks. Better start applying asap.