For me it has to be time limited with full features. I have found that it's not always a bad idea to give a fully featured product a longer period.

As I work in an environment that leans towards this weeks hot potatoe ends up as the neglected potatoe salad at the back of a students fridge the following week.

I have found that 14 days is just not enough time. On the occasion I have found one that does a long fully featured taster it has given me time to to use it feel the benefits and actually incorparate it into my working environment.

Then all of a sudden I get you have 3 days left. If it's then within budget and I can actually prove to those that hold the purse strings of it's real not just percieved benefits and you then have the sale.

This is all IMHO of course

Doug