open you webconfig file, and look for <system.web> tag. add this code or look for these tags sessionstate and httpRuntim.
sample tag
<configuration>
<system.web>
<!--
<sessionstate
mode="InProc"
cookieless="true"
timeout="60"/>
<!--
<httpruntime executionTimeout="60"/>
</system.web>
</configuration>
for more info, try to read and understand about the timeout (from sessionState) and executionTimeout(from httpRuntime)
sessionState[
^]
httpRuntime[
^]