[aqrashik]

I feel that limiting the number of uses may be counter-productive and may actually make users try it less so as to preserve the number of trials remaining and stretch out the trial.

Fixed number of days encourage people to try more knowing that their trial would expire whether they use it or not.

I have no data to back this up though and would be interested in actual results if anyone has tested the two approaches.

[judofyr]

What about sending an email like this a few days before the trial expires:

    Hey <name>, You signed up for our trial, but you've
    almost never used it. Click HERE to extend your trial
    for another month if you feel you need some more time.

[mikeleeorg]

I like this idea. You could also check to see if the user has used your product frequently within the trial period or not. An infrequent user could be sent such an email to give him/her more time.

This email also acts as a nice reminder for users who may have forgotten about your product. I've signed up for so many betas that I sometimes forget.