Send frequency changes the answer more than list size does. A weekly newsletter surfaces its own decay through bounces as it happens, so the platform's automatic suppression does most of the work. A list mailed twice a year accumulates a full year of rot and delivers it in one campaign.
Three triggers deserve a verification run regardless of the calendar. Before importing to a new platform, where a bad list damages a fresh reputation. Before a large campaign to data you have not mailed recently. After acquiring any list from a source you did not control.
The continuous half is a rule rather than a task. Suppress hard bounces on first failure, act on complaints immediately, and remove addresses that have not engaged in twelve months. Those run without anyone remembering to run them, which is why they work.
Old archives deserve a sample first. Take a thousand addresses from a two-year-old file and verify those. If more than a third come back invalid, the survivors are unlikely to remember who you are, and reviving the file whole is a reputation event rather than a campaign.
Verification and engagement pruning answer different questions and you need both. One finds mailboxes that cannot receive. The other finds people who receive and ignore you, and the second group is larger and costs you more.
Decay arithmetic on a 40,000-address list
Say you hold 40,000 business contacts and they decay at 25% a year, the middle of the usual range. That isn't 25% in one lump at year end. It works out to about 2.4% a month, compounding.
After three months, roughly 6.9% of the list has died. That's about 2,760 addresses. After six months it's 13.4%, or around 5,360. Mail the full list at the six-month mark without checking and your hard-bounce rate starts near 13%. That's more than double the point where providers begin throttling.
Set that against the price of checking. On our pay-as-you-go prices, 50,000 credits cost $89, which covers this list with room to spare, and unused credits don't expire. Most email platforms also charge by contact count, so you've been paying each month to store those 5,360 dead contacts.
How to tell whether you're overdue, or wasting a run
Your own reports can answer this faster than a calendar. Pull the hard-bounce rate for your last three campaigns to the same segment. A rate that climbs from 0.4% to 0.9% to 1.5% says decay is outrunning your suppression, and you're due.
Check the age of the oldest segment you plan to mail. If any part of the next send hasn't been mailed in three months, that part needs checking even when everything else is healthy.
There's a case where a full re-check buys you very little. If you mail the same list every week and hard bounces hold under half a percent, your platform is already removing the dead. Spend the effort on new imports and on the people who stopped opening.
New data is the exception to every schedule. A trade-show scan or a partner's list should be checked on the day it arrives, before it touches your main audience.
Related questions
How fast does a B2B list decay?
Commonly 22 to 30% a year, driven mostly by job changes. Faster in high-turnover sectors and at smaller companies.
Should I verify before every send?
Not for a frequently mailed list, where bounces surface continuously. Verify before sends to data older than about three months.
Is an old list worth verifying or discarding?
Sample a thousand addresses first. A third or more invalid means the file is not worth reviving whole.
Does cleaning replace a sunset policy?
No. Verification removes dead mailboxes. Sunsetting removes live mailboxes belonging to people who stopped caring.