Setting Domain NameServers (how long it will take?)

I've hosted with a couple of hosting providers out there and when I asked them how long it will take for my domain to take effect after set the nameservers, all of them told me that it would take up to 6 hours. Wow! I recently register a domain with Yahoo Small Business and setup the hosting nameservers. I was surprise that it went into effect instantaneously. I couldn't beleive it, but it happened. Maybe I was lucky! What do you think? Have similar experiece?