External IP address - expiration or changes?

I recently stumbled into the TM problem with SMTP sending emails - and having our TM Gateway and it's external IP address being blocked as per the internet Spam databases.   Rebooting the TM gets us a different external IP address and all is good for sending SMTP emails again.

https://whatismyipaddress.com/

Was wondering if our gateway external IP address expires after X number of days -
or how long are we able to hold the same external IP address ?