People deactivate and return the product, so we just check once per month. The ping off our server literally should take 2 seconds once per month.
We don't deactivate accounts when the serial is invalid, just in case it was some sort of error, but we do keep track and get in contact with people that don't correct the problem. Also without a regular check someone could copy the site once installed to as many servers as they wanted without us knowing.
It would be enough for honest users like yourself, but if we depended entirely on that I suppose we wouldn't need a serial at all.