You can make the changes in the Administration Workspace so the INTERNAL Workspace.

When logged in in your INTERNAL workspace :

1. Manage Instance

2. Security

3. Under Session Timeout, you can set the desired number

I think that putting 0 on the first makes your session never expiring, you can check it by yourselves by lookgin at the help text, or checking the documentation

These steps can also be done under the Shared Component menu on your applications.

1. Shared Components > Security > Security Attributes > Session Management

You could try calling these with very large values:

APEX_UTIL.SET_SESSION_LIFETIME_SECONDS

APEX_UTIL.SET_SESSION_MAX_IDLE_SECONDS