Interior designers often charge that way.

Make sure you run the design by folks here: Kitchen design is very specialized, and a general "interior designer" is not likely to know what goes into a functional kitchen.

I would suggest you get a floorplan with measurements and family requirements. Then post it here as soon as possible. This way, you will know how to direct the designer towards your desires. It would save you a lot of time and potentially money.

Designers, like financial advisors, should be paid in such a way that they have no incentive to drive you towards too expensive or otherwise poor choices. Per hour, or per job, never tied either to the cost of the job or to purchases that benefit the designer. Outcomes are much better when you can avoid conflict of interest situations.

Like all professionals, different designers have different strengths. Do you value yours for her aesthetic sense, or her ability to make good use of space, her attention to detail, or for other abilities like sourcing? If you only need her for aesthetic choices, maybe you could just pay her to choose colors, finishes, etc. Paying someone to learn about making good use of space in a kitchen, including NKBA guidelines and "ice, water, stone, fire", would be expensive. Planning a kitchen without that expertise could be worse.

That's how I paid mine though I doubt she actually charged me for every hour of time spent. Had I not purchased cabinets, cabinet hardware, counters and appliances through her she might have. My KD was a kitchen and bath specialist - not a generalist. She was fantastic and worth every penny. We had a difficult space to figure out - not the usual big square/rectangular space. That was the main impetus for using a KD but the flawless execution and outcome with no unpleasant surprises was also a huge benefit.