That's easy. It's the same as the answer to the question (women will get this) how much make up is too much'. Bobbi Brown's answer when asked by Oprah was, “When you look really good, you should stop.”
It's the same with kitchen design--except for one other thing. You stop designing/spending when you have a kitchen that works. That is, when you have a plan that makes your kitchen into a work space that feels like a play space, an organizational masterpiece, a jaw dropping beauty (on your terms) and the place that you love being in. That's how much you should spend on a kitchen.
Time Is Money
Think of it this way. If you plan to spend as little time as possible in your kitchen, you'll accept something off the shelf. That is no frills, no options, one size fits all. Like this:
You can like the look or not like the look. What's important to note about this "off the shelf" kitchen is that the basics are all there. You have everything you need and nothing more.
If it fits--great. If it's a bit under sized or under functioning, you'll live with it. It's probably not a kitchen design you throw your heart and soul into. Probably you would spend not a penny more than necessary to purchase this kitchen.
Here's the same kitchen (Chevy Chase, Maryland) after the Gilday designers got their hands on it:
Doing It Your Way
The homeowners had some very specific issues with the original kitchen. It was windowless and fairly featureless, yes indeed. More importantly, it didn't work the room efficiently. Although we slightly repositioned the kitchen in order to gain a couple of windows, it isn't much larger than the original. But due to clever planning it offers better storage, more work area and more seating space than the original. That's the benefit of taking the custom renovation approach to kitchen design. You get what you pay for and then some.