it varies by industry and traffic.

It's better to think about your goals. what are you trying to convert when the traffic arrives at your website? Joining a mailing list, adding to cart, buying a product? Contacting sales? How many of those leads do you need a month?

then work backward in the funnel and see how much google traffic you need to make that happen.

And remember google is a shrinking segment -- other social media channels will likely have a bigger impact.

websites aren't driving a lot of action these days.