Good question! I just purchased bulbs today. Here's what I found via Google.
http://www.uvm.edu/pss/ppp/articles/paperwht.htm

Check the instructions that come with the bulbs. Most paperwhites on the market have been pretreated and are ready to grow when purchased.

If not buying pretreated bulbs, then plant and put in the fridge or the cold cellar for 4 to 8 weeks until they start to show signs of growth, then bring into the warm.

Paperwhites don't need a cool period - it's one of those common misconceptions. You can grow them right away. Give them lots of light though, otherwise they'll get really floppy. You might want to grow them in a tall vase or stake them somehow since even with good light, they can flop over.

I have had them bloom in as few as four weeks, and as long as eight. Each set I but tends to bloom at the about the same time, but they will vary from year to year. If I want to be sure to have some for a particular time, I'll buy extras and start them a week or so apart - it's not like having more flowers is bad :-)

BP