.COM and .NET root zones are updated every 5 minutes, so as long as your registrar publishes to the root frequently, the domains can be live very quickly. This used to be a twice-per-day publish, but the powers-that-be changed to 5-minute updates about a year ago (I think? has it been that long?)

.ORG has been fast for some time. .BIZ is also very quick.

Other TLDs may still take 24-48 hours.