Historically there has only been a
perpetual license for Splunk products which does not ever expire. A few years ago Splunk added a
term license option for all products (including core and ES) which expires in a year. It depends on what you bought.
It should be clear if you log into you License Manager node and check your licenses.
It depends on the type of license your client has purchased.
If he bought a perpetual license it will not expire. A term license is valid for the purchased term only.
For the Splunk Enterprise license you can check the validity in the licensing section of the Splunk frontend. As the Splunk Enterprise Security License is only a permission to download the current app, you won't find any license information inside the Splunk installation, but rather in your account information on the Splunk website - check https://www.splunk.com/my-account/#/
Yes, Enterprise Security license could expire.
Splunk offers both Perpetual and Term Licenses for Splunk Enterprise Security. A Perpetual License is a one-time license fee that grants you an indefinite right to use the software for as long as you comply with all terms of the license agreement. A Term License is for a specific time period, usually a year, during which you are allowed to access and use the software. At the end of the term, you must stop using the software or purchase new licenses. Splunk also offers multi-year term license options for customers interested in a longer term commitment.
There you go for more info: https://www.splunk.com/en_us/software/pricing/faqs/cyber-security.html#Splunk-ES