Ask HN | How you guys make Time Estimates

I was wondering if you guys could share your knowledge and experience on estimating the number of hours a software project will take. I do freelance RoR work and many of the clients ask for an hourly rate and an estimated number of hours. I find my self totally confused when i try to estimate hours for a large and complex enough project. How should one go about this ?? thoughts , experieneces, suggestions please.