DNS timeout value

After an Instance Refresh or a Site Switch, we recommend that customers set their DNS timeout value to 5 minutes. If customers are following our infrastructure best practices by not restricting access to Salesforce IP ranges, allowlisting our entire range of IP addresses (if using IP blocklisting) and setting the DNS timeout value to 5 minutes (default setting), a site switch or Instance Refresh should be seamless to users.

If a customers network or IT group controls the DNS timeout values, then they may need to refresh the DNS cache and restart any integrations following the maintenance.

The DNS timeout setting is not a Salesforce setting. This is a setting that a customers IT/Networking group might manage on the local network. We do recommend a shorter TTL (Time To Live) value, so that when the new instance comes online, the network will have the latest DNS information to ensure it can find the new location quickly.

Time to Live values are always represented in seconds. Most DNS setup configuration services provide you a preset list of values to set your records to.
300 seconds = 5 minutes = “Very Short”
3600 seconds = 1 hour = “Short”
86400 seconds = 24 hours = “Long”
604800 seconds = 7 days = “Very long”

The DNS timeout setting is referring to DNS server resource records with a Time To Live (TTL) value of 300 seconds.

Salesforce sets the TTL for 300 seconds (5 minutes) on the resource records that they are the authority on and the recommendation is that an upstream caching server (for instance, one within your enterprise infrastructure) should also use this value and not cache these records for more than 300 seconds. 86400 seconds (24 hours) was common in the past. Caches should normally use the record's TTL. If you decide to set your own cache, then use *.salesforce.com, *.force.com, *.documentforce.com, *.visualforce.com, *.lightning.com, *.salesforcecommunities.com, and possibly *.cloudforce.com if it is in use in the organization.

In short, following best practices by setting your DNS timeout limit to 300 seconds, will limit the chances of users noticing the maintenance that has taken place.