Not lifetime value, but first contract amount.
If it's a month-to-month contract for $1000, that's the opportunity amount.
It it's a 12 month contract (Regardless of billing frequency, annual or monthly), then the oppty amount is $12,000.
If it's a 3 year contract (regardless of billing frequency) it's a $36k oppty.-- Thanks Rob.!
In this situation, I would use the average subscription length. You should be able to figure that from mining your data OR sending a survey to your customers. I think LTV would be a waste of time figuring out this early on for a startup.