Discussion: How much traffic should my website handle?

How much traffic should a small website be able to handle before experiencing degradation?

For larger websites a flat +100% above peaktime traffic (or something like that) might work, but if I'm getting 50 daily visitors it doesn't make much sense (because getting double that can happen fairly often and is easy to optimise for).

I guess this is a "What's the optimum level to scale for" question, any opinions?