How long does it take to process an e-Visa for Thailand, and what are the entry timeframes?

Evisa is usually less than a week if you submitted everything correctly the first time.

There's 2 time frame you have to be aware of with a Visa. I'm going to use Tourist Visa as an example since you didn't tell us what type of Visa.

A tourist visa is valid for 90 days from the day it is issued. That means you have 90 days to enter Thailand. When you enter you then receive 60 days inside Thailand stamped into your passport.

So if you enter 5 days after the visa is issued, you'll receive 60 days in Thailand.

If you enter 89 days after the visa is issued, you'll receive 60 days in Thailand.

Don't confuse the expiration date on the visa with the date you are allowed to be in Thailand. That date will be on the stamp that immigration gives you when you enter the country.