how to set expiry date for user subscription?

Well that completely depends on your application. It's pretty hard to push you in the right direction without much information...

If you have a paid subscription you also need to cancel their payments. However if you just set a date on the user object or subscription object and check on that date you should be fine as well. You can also run a cronjob to automatically turn these subscriptions into inactive for example when they reach that date.

when user becomes a premium member, you will know what package they subscribed to so you can set the expiry date accordingly.

if you are using stripe/braintree then try using CASHIER (theres laracast videos on it too) that makes it all very easy to work with subscription model