It’s not the validation that you need to do regularly, that just checks if it’s still valid or not.
The code you get from the OAuth process is single use, but what you receive as well as the Access Token is a Refresh token. This refresh token can be used get a new Access Token and Refresh token as explained here: https://dev.twitch.tv/docs/authentication#refreshing-access-tokens
User Access tokens last roughly 4 hours, so if you need to have a valid token at all time you can just go through that refresh process a little more frequently than 4 hours, or if you don’t need an access token valid at all times you could go through the refresh process only when you need to.
As for throttling or anything like that it shouldn’t be a concern if you use the API sensibly. For example all endpoints use caching, so if you’re wanting to repeatedly check the same endpoint then once a minute is considered the standard minimum interval.