When should I pay a referral reward?

The simple answer is: As soon as the placement is made. 

BUT - What if they fail their probation

Yes, this happens but best practice is to account for this in your referral rewards. If your referral rewards are typically $500 and you have 'hypothetically' 10% of candidates fail their probation, then adjust your referral reward by 10% to $450.

This means your referrers don't have to wait and wonder if you are going to pay out, and they will refer to you again quicker than if you make them wait 6 months for a placed candidates probation to be complete! 

RESULT:

1. Same amount of money spent on referral rewards

2. More engagement in your referral scheme by referrers