The answer to this question lies in your site and your traffic. In my own case, my site (at
http://www.girlsdotcom.com) features my own photography only. It is strictly a soft/medium-core site. Never a guy on it...ever. Girls solo, girls using toys, girls having sex with girls. It's for guys who aren't interested in looking at hard-ons, in other words.
I charge $19.95/mo recurring and $21.95/mo nonrecurring. I have members who've been members more than a year. I have members who go from buying memberships a month at a time (recurring or nonrecurring) and then decide they like it so much they buy a 3-month membership ($55). I do have a 3-day trial for $3.95, which I'm thinking of dropping, since my tour actually shows a LOT of my content (in lower resolution than the material inside).
Once inside, they find an update schedule. I tell them what's coming up. Scheduled updates are every six days. Once they are members for a while, they notice that I always give them one or two surprise updates between the scheduled updates. I feel that this is important to keep them on their toes and to make them think twice when they think about terminating their membership. It's important to note that the surprise updates are very good material...just as good as the regular updates, and sometimes better. Never disappoint and keep them guessing is how I feel.
At any rate, how much you charge is set by the market, not by those of us who are replying. Just as you always need to try new things on your site(s), so you need to experiment with pricing.