Time is the one thing we never seem to have enough of, and you can't ever make more. So based on the laws of supply and demand, your time is extremely valuable. How you charge for your time depends on a few important things.
First things first, do your research and find out what the going rate is in your area. Don't forget to find out how long your competition has been in business and what type of design they specialize in? Do they charge hourly or by project? Are you really comparing apples to apples?
Secondly, what is your specialty? Are you a re-designer, someone who rearranges the furniture your client already has? Would you like to design a plan for someone's room and let them do all of the leg work, like suggesting paint colors and floor plans? These are consultation-based design services and I would recommend charging a slightly higher hourly rate or a flat fee based on the room size, or square footage.
The trick here is having a well thought out plan based on your income goals for the year, and then making sure you have a strategy for finding those clients. As a consultation only designer you are basically working dollars for hours. That means you have to work enough hours in a week to satisfy your expectations.
If you are planning to actually purchase the furniture for your clients and place orders, handle the delivery, and deal with returns, then you are a product driven design consultant. You should be charging an hourly fee and a percentage of the products you use in the space. This money can either be a service fee you charge the client, or a designer discount offered by the supplier of the product.
There is a lot of extra work involved in tracking special orders, so you must charge for the additional responsibility and liability involved in this type of design. I'm not trying to discourage you from being a product driven designer. It can be extremely profitable. Just be aware that you must be much more organized, and have excellent attention to detail to be successful as this type of designer. (Learn how to overcome these challenges in my Design for a Living Virtual Bootcamp.)
Last but not least, how experienced are you? If you're just starting out in this wonderful world of interior design, then giving your clients a discount can be a great way to build up a base clientele. Be sure to let them know that they’re receiving an introductory offer and as your popularity grows, so will your fees. There isn't any one, right way to charge for design time. There's a fine line between underselling yourself, and scaring off potential customers.
It's important to start off with a solid plan for your business. If you change your tactics at the wrong time, you run the risk of getting a reputation as a wishy-washy, unprofessional designer. Build your business with a solid foundation and you'll enjoy years of happy clients and healthy pocketbook.