Depends on how much you are trusting them with. Any amount of trust comes with the possibility you will get burned. The higher the value, the less trust you should have.
$5 - Most people.
$50 - People that seem honest, have some short history of being honest.
$500 - People that have a well established history of being honest. (Last stop for anonymous transactions.)
Edited out the bits I didn't want, but the sentiment is spot on. For anonymous transactions, how do you feel about it if it was a gift or donation where you don't get the money back.
If I give $5 to someone on the street I wouldn't expect it back - especially if they need it for food or a place to sleep.
$50 is more donation territory for a lot of people, but getting it back eventually or some other way is nice.
$500 or around 100 bitcoin - never hurts to ask and there is some small beginnings of a trust network/grading system evolving.
It also depends on how much you can afford to lose. As I've gotten older, the amount increases.