Unlike other FFP, LH M&M miles can not be extended simply by account activity. From
LH M&M FAQ:
For how long will my award miles remain valid?
Award miles remain valid for 36 months from the date of the activity and expire — if they have not been used — at the end of the quarter. Members who hold the Frequent Traveller, Senator or HON Circle Member status or who have a German Lufthansa Miles & More Credit Card* are not affected by this mileage expiry deadline.
(*For customers with an international Miles & More Credit Card special terms and conditions apply.)
The miles due to expire at the end of the current quarter and at the end of the next quarter are shown on your online statement.
You will receive a summary of all the miles which are due to expire at the end of the current quarter about two months in advance in your online newsletter.
As an illustration take the following example: you flew from Frankfurt to New York on 12.12.2015. The miles you earned for this flight will expire after 36 months at the end of the next quarter, i.e. on 31.12.2018.
If you wish to prevent your miles expiring by applying for a Miles & More Credit Card, you can do so here.