How much CPU use is too much?

In my opinion, this question could be asked as how long is a piece of string? Obviously when you hit 100% CPU use everything else is going to suffer. Personally I investigate when my server's CPU remain above 50%.
Your graph shows bursts not even as high at 50% so I think your server is in good condition, you would need to take note of how many websites are running and/if any DB's are running and the sizes of them. There are alot of factors which could effect your server, but I think CPU usage isnt one for you at the moment.

Looks fine...You don't want to be tooling along flatlined: that means you've paid too much for your hardware. And if it's topping out at 50%, then you've got plenty of spare capacity.

The only thing I'd worry about is that Tuesday spike...Looks like you handled it fine, but spiky traffic is the worst to plan for. What kind of site is it?

It seems your graphic sampling interval is around 10 minutes or so. That means during the Tuesday spike you might have reached 100% CPU load during a significant period of time (eg: 1 minute). That said, CPU load, while interesting to see how the load evolves during time isn't that useful in identifying CPU bound systems. I highly prefer watching the run queue size metric.