You're making the mistake of not thinking like your customer. Right now, you're approaching the problem from the position of "how can we stuff our bank out the fastest?" but you should be asking "what is most appropriate for our customers?".
When a potential customer is deciding whether to subscribe to a service or not, they're considering ROI and risks:
- For how long will I need this?
- How quickly will I get some value out of this?
- If I subscribe for 3/6/12/24/etc months will I continue to receive benefits after X time?
- Do I have the funds to pay for 3/6/12/24/etc months at once?
- What will happen if I no longer need the service but it had been pre-paid for X time ahead?
- What will happen if the company no longer provides the services but it had been pre-paid for X time ahead?
So when you're devising your payment schedules these questions should your top priorities. Otherwise, you'll end up offering $200 annual subscriptions to a low-value service needed only a couple of times a year and wonder why you can't sign any customers.
@bangdang has made a good point that sometimes monthly subscription charge is too low and isn't financially feasible so you have to charge in larger packages. However, it all still comes back to the value and the risk: if you pre-paid for a year of $2/mo. service ($24) and the company folded after 8 months, you lost only $8, a negligible amount, as oppoosed to $80 or $40 had the service cost $20/mo. or $10/mo.