That depends on the TLD. Mainly it depends if the registrar you are using immediately processes your update. For com and net the registrar publishes the whois themself, so there is no general answer. For all other gTLD's it's more or less real time. There are many whois sites / scripts which catch the information and therefor you could get for some time old information not because it's not updated but because it's not refreshed in the catch.
If it's cached then you can change the way you look it up, if it's a com or net domain and it takes long, then you can change registrar. If it's an other gTLD and it takes long then your registrar doesn't process updates without delay and again the only solution would be to change registrar.
It depends on what kind of whois info you are talking about. For mandatory fields like nameservers, it's done in SECONDS (at least for .COM and .NET). And these days, any registrar which does not submit changes to the registry right away (in less than a couple of minutes) shouldn't be in the business at all. Even for other fields like Admin Contact email, I don't see how and why it is not updated in batches of less than a few minutes. Anything longer should only be occasional problems; or else, you may want to transfer your domain away.
Co-Founder@HostHideout. Profoundly influenced by #Bauhaus, @Nameslave unrepentantly embraces #Minimalism with a bias for functionality, color theory and pixel precision: a #multimedia messenger in the McLuhan sense. His totally irrelevant M.Ed. dissertation examines Organizational Culture and Change Management. He also likes Patrik Ervell, Wong Kar-wai and IKEA.