When it comes to jweb, If you don't configure custom idle-timeout, the default timer will activate. By default, the session idle-timeout is set to 30 minutes, meaning if there is no activity within that time frame the session will timeout. 

In order to set a custom idle timeout for jweb, Use the following commands : 

[edit system services web-management session]

root@SRX1500# set idle-timeout ?

Possible completions:

 <idle-timeout>    Default timeout of web-management sessions (1..1440 minutes)

https://www.juniper.net/documentation/us/en/software/junos/user-access/topics/ref/statement/session-edit-system.html