The unit should be milliseconds (however @eyesnz has not found that to be correct). According to the MSDN DataServiceContext.Timeout Property page: 
  This value is passed to the Timeout
  property of the underlying
  HttpWebRequest object. The value must
  be set before executing any query or
  update operations against the target
  data service can affect the request.
  The value may be changed between
  requests to a data service and the new
  value will be picked up by the next
  data service request.
Referring to the MSDN HtppWebRequest.Timeout Property page:
  The number of milliseconds to wait before the request times out. The default value is 100,000 milliseconds (100 seconds).