It really depends on the niche you're in, and what you can afford based on a reasonable "drop-off rate" for your subscriptions.
For web hosting subscriptions, there are many merchants offering 100% or more of the first month's hosting fees. A few require pre-payment for 3 months and offer more than 100% of the first month's fees.
You might also consider a recurring commission (such as 25% of each months' subscription fees, for as long as the subscription continues). This can be a huge accounting and technical issue.
You should also be very clear about any "contingencies" -- if payment is not based on the first month's payment but is actually contingent on renewal payment for the second month, say so.
, someone else announced a subscription-service (website monitoring) affiliate program today (http://forum.abestweb.com/showthread.php?t=115500
), so you might want to monitor that thread to see any affiliate responses.
The first place to look, always, is at your competitors.
What are they offering? How do their affiliates respond?
One of the most common complaints about affiliate programs connected with "subscriptions" is that there are frequently very high reversal rates (due to cancelled subscriptions, credit-card disputes, and fraudulent orders).
It may also be helpful to identify "aligned non-competing merchants," to seek out advice. For example, someone selling website-monitoring services would absolutely want to talk to some web-hosting resellers who operate affiliate programs, to get some advice.