Hi there,
Thanks for your suggestion.
The longest billing cycle for a recurring product in WHMCS is currently 3 years. But if your SSL issuer offers it, you can potentially sell it as a one-time product instead.

However it is my understanding that the maximum validity period of SSL certificates is new being limited to a little over 1 year due to recent changes by Google/Firefox and Apple in their browsers. Most SSL issuers have followed this advice and adjusted their maximum length accordingly.

I'd be interested to hear which issuer you're using that is offering certificates for up to 6 years, and how they expect to overcome these browser-imposed limits?