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That very much depends on the company size too. If you're in a 100 person company you'd need a very high referral fee before you start pissing off your colleagues referring crap candidates. Whereas if you're in a megacorp you can pretty much do it with impunity.

One thing to keep in mind with the internal referrals is that people tend to recommend those who they like or with whom they had had a good "cultural fit" before. So if you only rely on internal referrals you might end up having a very homogenous workplace.

I've never seen a referral bonus come without a requirement of te new employee staying at least 6 montha or a year.

In most places, less than 6-12 months implies a pretty serious mismatch of expectations of some sort.