Common Record Types
A or AAAA Record – Usually a 1 hour TTL is a good compromise between enabling fast changes while taking advantage of DNS caching while someone is visiting your site. If changes to this record are often or need to happen quickly in an emergency, you can usually set it as low as 30 seconds. For DynECT Managed DNS features such as Active Failover, Load Balancing and GSLB, you can set the TTLs between 30 seconds and 5 minutes. For non-critical records that rarely – if ever – will need to change, you may be able to get away with having a TTL in the 12 hours to 1 day range.
CNAME record – In many cases, a CNAME record will never be modified (ex. pointing www.example.com to example.com’s A record). In those scenarios, a 12 hour to 1 day TTL is a good compromise as the benefits of caching outweigh need for a faster propagation time. If your CNAME record could potentially change (such as if you are using a CDN), you will want to a have a lower TTL.
MX Record – MX records rarely, if ever, change, especially if you are using an email provider with a good track record or you have lots of redundancy when self hosting. You can usually set this to a 12 hour or 1 day TTL. If you want to ensure faster propagation times in the event of an emergency, a 1 to 4 hour TTL is a good compromise.
TXT Records – Most commonly used for SPF or DKIM records. Usually safe to set in the 1 hour to 12 hour range since they rarely change.
In the end, keep in mind that what you set the TTL to is what you are most comfortable with. It is all about striking a reasonable balance between a fast propagation time and taking advantage of DNS caching.