It depends on the client as well as the timeout setting on the proxy class.
If the client is a Windows client then in theory, a 30 minute timeout will
not be a problem. If the client is a Web form then this will almost
certainly be a problem because although the call is asynchronous, it must
return before a response can be sent back to the client. There is an
alternative: if you use the DHTML Web behavior, which makes Web service
calls directly from client-side script, then these calls are truly
asynchronous, and you can use the Web page while waiting for the response. I
have used this behavior extensively. It is not suitable for all purposes,
but is especially good for validation-oriented Web service calls.

Jeffrey Hasan, MCSD
President, Bluestone Partners, Inc.
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Author of: Expert SOA in C# Using WSE 2.0 (APress, 2004)
http://www.bluestonepartners.com/soa.aspx