I'm new to freelance designing for clients, I normally did things just for myself or for friends when they needed something done and I had a free minute. I've also worked at a retail store that does "desktop publishing" and offered a 24 hr turnaround guarantee.
Because of this, I've become accustomed to working really fast. I still plan ideas and do research, but I usually go with my gut feelings on design direction (usually the second or so idea I have is the strongest in my opinion) and the clients I've worked for always love the work I do for them.
But my problem is, because I work so fast and can almost pump out the designs they need, how do I should I charge?
Charging by the hour would be a huge loss for me. But I don't know what to charge for a flat rate since I don't know what the "average" time for a design should take (if there is such a thing).
When doing research, everyone always just says "It depends on the designer." But I feel I could be making a lot more money for my skills, I just don't want to lie about my time.
The hourly rate, in freelance environment, is often used for some routine tasks that "must be done" but require less creativity. In these activities, you can more or less expect the time the task will take and thus the final price.
In design or other highly creative domains, this doesn't have much sense as what counts is the "idea" and not necessarily the execution. It is also much more difficult to correctly estimate the number of hours it will take.
In your situation, I can say: go for semi-flat rate. By that I mean "price per project ". You can have a +/- flat rate per project type, but you can adjust it because of some specificities (so no real flat-rate, same for every client/project).
In your case, you can perhaps charge more for faster delivery. E.g. average "industry time to finish the task X" = 2 days, if you are able to do it in one day, give the client the possibility: delivery in 2 days for $Y or in 1 day for $Y + 50%...
And concerning the prices, just do some market research. Even if designers often give estimations only for specific project, just invent one such a project and ask them for price (or ask someone to ask them as a potential client). Do it for the maximum of your competitors and you will have some basis to estimate the average cost...