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/
Josh is right, it definitely depends on your business situation.
We usually measure our lead scoring performance based on a blend of feedback from sales, funnel contribution, opportunity conversion rates, and opp win/loss ratios.
How often you change your scoring model really depends on your sales cycle. You don't want to make major changes too often/consecutively within your business's typical sales cycle or you'll have difficulty measuring its performance accurately.