If you have not bought anything from LEGO in a while, check your balance. Points are not lost on a fixed date after you earn them. They expire when your account goes quiet for 18 months.
How the 18-month rule works
LEGO's help page says Insiders points "will only expire if you stop using your account for 18 months." The clock is about your account activity, not about each individual order.
Anytime you earn or redeem points, the timer resets and you get another 18 months to use them. As long as something happens on your account inside that window, your balance stays.

Download flowchart
What counts as activity
Earning points and redeeming points both reset the timer. Earning happens when you buy from LEGO.com or a LEGO Store while signed in to your Insiders account, or when you register a set. Redeeming means spending points on a reward.
If you are unsure when you were last active, open your account's points history on lego.com and look at the date of the most recent entry.
What happens when points expire
When you are close to losing your points, LEGO sends an email to the address linked to your account. That is the warning, so keep the address up to date and check your spam folder.
LEGO's help page does not describe a way to restore points once they expire. If it happens to you anyway, contact LEGO customer service and explain what happened. Do not count on it working.
How long do discount codes last?
That is a different clock. When you turn points into a discount or a reward, LEGO gives you a code, and LEGO's help page says codes are valid for 60 days. If a code runs out before you use it, it stops working, so redeem when you are ready to check out. If something goes wrong, see LEGO points won't redeem?
How to keep points active
Make a small purchase or redeem a reward at least once every 18 months. If you are sitting on a big balance, use the points calculator to see what it is worth in your currency before you decide what to spend it on. A few examples of common balances: 1,000 points, 5,000 points and 10,000 points.
This page is about LEGO Insiders points only. Points from other retailers' loyalty schemes, such as Target's, have their own rules. LEGO Certified Stores and other retailers don't earn or redeem Insiders points at all.
The same help pages say points cannot be shared or transferred to another person. More on that in Can You Transfer LEGO Insiders Points?