DNS propagation is the process of DNS settings (like nameservers) updating around the world. Each user’s browser will pick up changes at different time intervals based on the ISP they are with.
Generally DNS is refreshed every 2-4 hours. A lot of hosting companies will advise that DNS propagation can take up to 48 hours. It can take as little as a couple minutes, but generally you will see the changes between two and four hours.
When editing DNS settings, you will see a TTL (Time To Live) entry. The TTL is meant to advise how frequently the DNS should be checked for new records – please be aware that TTL is a guideline and isn’t always followed.