This is the acceptable use policy for the WHOIS.  

The acceptable use requirements are:

• The maximum query rate is 5 queries per second with a maximum of 1,000 queries per rolling 24 hours.
  
  
  
• If you exceed the query limits a block will be imposed. For further details regarding blocks please see the detailed instructions for use page.
  
  
• These limits are not per IP address, they are per user.