There is no rule in the UCC, UCP or ISP which prevents this, however, each bank will have their own policy as to whether or not they allow this. Most, if they do allow it, will only extend, for example, within 30 days of expiry and then a new LC would be required to be issued (new application, possibly new reimbursement agreement, etc...) If there is a confirming bank, it would be best to understand their position prior to extending as they would not be obligated to automatically extend their confirmation.
It can be extended beyond the expiry date but subject to the consent from the parties involved i.e. beneficiary, confirming bank if any......
there is no specific time limit to the extension if accepted by the parties concerned.