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).