Hello @BethanyW1127 ,
It can be whatever you want, the main things to keep in mind are:
What's your servers' capacity for handling updates and changes, if necessary
How often are changes occurring on your end for your DNS records?
How quickly do you want systems that are linked to your DNS records (such as your account with us) to be able to also reflect those changes. This can impact your sending, as having mismatched records due to longer delays can result in your scheduled sendouts erroring.
Generally speaking, the TTL can be longer as far as we're concerned, to reduce your server load. For example, if you're using CNAME then anywhere from 12-48 hours is pretty standard.
See also:
Email Authentication FAQ
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