Acceptable CPU usage for 4 core 3.4gHz i7

Solution 1:

Zero is Ideal

Ideally, your kiosk application should use 0% CPU when not actively being used. It is unlikely you will achieve this without considerable effort but even a target of 5 – 10% use is good.

At 150% constant CPU use, the application would likely be considered faulty by Apple's Mac App Store reviewers – and by many customers.

Hints: Offload to the Right Framework

If your kiosk application is animating or showing moving content such as adverts or movies, do as much as you can with AVFoundation/QuickTime. This will typically offload the processing to the GPU and allow the CPU to idle.

For other animations be sure to adopt CoreGraphics and CoreAnimation. They will dramatically improve your application's energy impact.

Energy Efficiency Guide for Mac Apps

Study Apple's Energy Efficiency Guide for Mac Apps. The introduction sets out Apple's intentions:

Your Obligation as a Developer

Even small inefficiencies in apps add up across the system, significantly affecting battery life, performance, responsiveness, and temperature. As an app developer, you have an obligation to make sure your app runs as efficiently as possible. Use recommended APIs so the system can make smart decisions about how best to manage your app and the resources it uses. Whenever possible, avoid unnecessary updates to the user interface and I/O. Power-intensive operations should be under the user’s control. If a user initiates a large iMovie render, Automator batch job, Compressor conversion, or Xcode compile, for example, the user should not be surprised if the activity consumes power. Strive to make your app absolutely idle when it is not responding to user input.

Use the developer tools to profile your application's energy impact and ask away on Stack Overflow for ways to reduce CPU load. Good luck!

Solution 2:

150% of the total CPU would, of course, not only be totally unacceptable - but physically impossible.
I know most sportsmen seem to think that they can give at least 110% every time they are interviewed, but no-one pointed out the simple facts to them, I guess ;-)

Activity Monitor shows individual applications' CPU usage per core [including virtual cores], so 150% is actually (for an i7, with 4 cores + hyperthreading) 150 / 8 = 18.75% of the total machine capability.

The System + User total underneath the individual figures is a more accurate representation of the overall machine usage & a quick tot-up of the figures gives you a rough idea that this is in fact the true state of things.

Taking this as an example, the visible percentages add up to about 120, yet the System + User figure is only 5.3%
I have 24 virtual cores [2 x 6-core CPUs + hyperthreading] so 120 / 24 = 5 [close enough to account for the processes further down the list]

At 18.75% of the total machine's capacity, I don't see it being worth the effort to optimise - unless another process needs any significant portion of the remaining capability.