You don't have to set it when setting up the channel. You can keep it open infinitely if it benefits you.

Within LN+ I will add related info to this page, so you can easily keep track of how much longer you must keep a channel open: https://lightningnetwork.plus/my_swaps

Thank you 
But what's the reason of field  "Channel Duration" ? I
f for example  it sets 3 months
in case if i closed after 2  month or 4 month i will be charged for same amount of sats

It is the minimal amount of time that you and everybody  on the ring expect to maintain the channel, but there is no mechanism of enforcement. 

ok, thanks