Your question is a little vague. What do you define as domain propagation time? DNS changes? Domain registration? Nameserver updates?

For registration and nameserver changes, most processes are entirely automated these days, so changes can occur instantly or in a manner of hours. As an example, registering a .com from Namecheap in my experience means it becomes available instantly. Nameserver changes again occur instantly.

DNS propagation however is vastly different. Because DNS can be cached at several places, it's typically advised to wait up to 24 hours for propagation to occur. Some ISP's (AOL) can cache DNS for up to 72 hours in extreme circumstances, so to be completely safe it's recommended to plan around 5 days in advance for complete propagation.