There is a file called web.xml somewhere in your BI Publisher install tree. Per the Oracle blog post, it's found at:
\applications\xmlpserver\xmlpserver\WEB-INF\web.xml
However, on my BI Publisher Trial Edition installation, it's found at:
\bip\server\wls\user_projects\applications\bipdomain\xmlpserver\WEB-INF\web.xml
Thus, you may need to do a bit of searching to find it. Wherever it is, you want to find this tag combination:
<session-config>
<session-timeout>20</session-timeout>
</session-config>
The value of session-timeout is measured in minutes.