The default is 48 hours as setting this to anything higher would put a huge strain on the CMS and Player as you are asking the CMS to calculate forward 3 months worth of schedule, and the Player to download and consume that, which would cause issues with a relatively simple schedule.
is this new update? because before this i using Version 2.3.0 cms. it can run 3 months forward. but now i upgrade to new version. it doesnt support it? or do i need to downgrade it back ?
Extra validation is provided to protect both the CMS and Players, as previously mentioned in my earlier response, setting to 3 months is something we would not be advising as we cannot be certain that it would be issue free. You can of course rollback to an earlier version, however please be aware that you would not benefit from security updates Supported Versions