I'm currently getting about 35% of my traffic from Google, and I've heard some websites get over 70% from Google. It really depends on how much traffic they're sending you, and where your other traffic is coming from. If you have an advertising campaign going on another site, your percent of search engine traffic will be lower.
I think anything over 30% search engine traffic is pretty good. But it'll start off a lot less for new sites. It took me a few months to get over 30% traffic from Google.