For lists, which exceed the 10,000 contacts mark, we suggest verifying it every 2 weeks. Smaller lists can get away with monthly check ups.
This is because email addresses are often changing and from statistics available, we can say users continually change their email addresses and providers for that matter.
Sending to invalid emails will not only render no results but also put your reputation on the line. Higher bounce rate would mean lower deliverability rates and worst - rejection of all emails coming from your server or address.