Hacker News Reader

You want it to be long enough for your customers to integrate it into their habits. If they tend to be slow to decide (low conversion for a given trial period length), then they might just need more time to depend on it.

Two weeks seems fair. Long enough to get familiar with the system, but short enough to make it inconvenient to keep signing up for free trial periods repeatedly with different twitter accounts they can keep making up.

FWIW, Basecamp offers a 30 day free trial for their services.

you want to do some a/b testing ;)