how can i increase Session time out for 12 hour in asp 2.0

Asp.Net Session Management has two Modes.

InProc

Loss of Session is a common problem in this mode. IIS keep recycling the worker-process after some duration, which causes the loss of Session Data.

Have a look at below link for more informaton on this.

http://blogs.iis.net/webtopics/archive/2009/07/21/in-proc-session-state-management.aspx

Below CodeProject article has a work-around for this problem.
Prevent Session Timeout in ASP.NET

OutProc

In this mode of Session Management, settings of web.config will work properly. Have a look at below link for more details.

http://msdn.microsoft.com/en-us/library/h6bb9cz9(v=VS.100).aspx

You need to increase session timeout in WEB.CONFIG file. the default session timeout time is 20 min. Set timeout in Minutes

<configuration>
  <system.web>
     <sessionstate timeout="20" /> 
  </system.web>
</configuration> 

Open the IIS, click on the Application Pools, Select the Application pool for your application.
Right Click --> Select Properties.

set the time which u want

You can do the same from IIS and from web config file as well, anywhere which is suitable for you,

From IIS :

Open the IIS, click on the Application Pools, Select the Application pool of your application.Right Click on it, Select Properties.In the Performance tab, Set the idle timeout.

Fron WebConfig :

<configuration>
  <system.web>
     <sessionstate timeout="20" /> 
  </system.web>
</configuration>

By default its 20 mins. Change it as per your needs.

hi, do u fix up d issue u face?? currently i also face this issue. If u have solution can share it to me?

Open the IIS, click on the Application Pools, Select the Application pool for your application.
Right Click --> Select Properties.
In the Performance tab, Set the idle timeout as your desired minutes for "shutdown worker processes after being idle for ..... minutes".

if it is timedOut while working it self, then check following points

1. have you use web services in your code ?
2. have you create any directory using your code ?

Reload an iframe on your page every few minutes using javascript. This will prevent session timeout. Make sure the page you load in the iframe is aspx, returns something different (maybe just the current time) on every load and that the request parameters are different each time.