Salesforce.com offers a 30 day free trial and it works for them. If you are offering a Wiki I believe it would be a good idea but in doing so you should be sure to engage the customer with daily emails over the 30 day trial.
When I tried offering my first web application in 2001-2002 with a 30 day free trial I has 1,200 trial sign up and 3 conversions. I didn't follow up with those who signed up, I didn't collect phone numbers to make any phone calls, I failed in that partly because I just didn't engage the consumer. I got them in the door but left them hanging to figure things out themselves and sell themselves.
Might also be worth ensuring that the users have support or at least howtos to set and use your app, ensuring that they're actually using it during the trial (maybe auto-extend their trial if they never got around to using it -- you don't want to lose them instantly if they finally go to the app and the trial is over).
I like that auto-extend idea. "We noticed you didn't get a chance to use your trial period, we wanted to give you a second chance trial and help you get started".
I know that my previous company did get more customers when they were advertising a free 30 day trial than whey they were not. I am certain that enough of the customers stuck around after 30 days to make it worthwhile.
The business was very much customer-service oriented. Every new customer got a phone call to ensure that they were properly set up, give them a 5 minute or so introduction to the product, and personally invite the customer to a weekly live training webinar.
They catered to less computer-savy people mostly, but even the technical folks seemed to appreciate the personal touch.
When I tried offering my first web application in 2001-2002 with a 30 day free trial I has 1,200 trial sign up and 3 conversions. I didn't follow up with those who signed up, I didn't collect phone numbers to make any phone calls, I failed in that partly because I just didn't engage the consumer. I got them in the door but left them hanging to figure things out themselves and sell themselves.