How to prevent Web service from time out - From the web service itself

dear all,
i have a web service that perofrms a lenghty operation.

this web service can process single client account or 10000 clients accounts per call.

each client account takes approx. 2 sec. to complete

so when the user asks for more than 50 clients , the webservice times out. actually the web service times out at the client side but the service itself completes execution successfully.

now what i want to do , is to insert a line of code in the looping code of the services ( in the start in each client cycle), this code should notify the calling client that the service is executing and processing the request ( somthing like application.doevents) .

this way the client calling this web service will not timeout when calling the service with long requests.

I dont want to change the timeout for the calling client application (which is a windows application) because we have hundreds of clients using the service and we cannot update all of their applications EXE. that is why we decided to use webservice in the first place !!!

i have searched a lot on the internet and i have read much about the messagepump but couldn't find anything related to webservices applications.

so is there anything available in the .net framework that allows me to do so or i am dreaming for something that is not existing in real life.

thx for your time and best regars