Hi Patrick,
It definitely depends on your situation. I usually watch this about once a month and then adjust the MQL threshold. I then reported on the number of leads which MQLd > Passed to Sales > SAL > Opps > Won
If you've had your model running for a bit, then you should be able to do some testing to see what behaviors, scores, etc are driving the Won$.
I have a couple of posts on this
http://www.marketingrockstarguides.com/how-to-build-a-lead-scoring-system-in-marketo-254/
which may help you.