The problematic word in your question is "accurately"...which is kind of the opposite of "estimate".
Designers are notorious for not knowing when to stop, and everyone from students to junior designers to the most seasoned professionals are guilty of this.
One way to estimate hours is from previous projects, even small ones. For example, I know I can do 2-3 good concept sketches an hour (8.5x11 paper, a couple pens). If a client wants 20 ideas then I'd estimate 8-10 hours for just the sketching, and add on a few hours for research, poring over magazines, and general brain-wandering time. As projects get larger you can blur these lines a little, and always pad the hours if you want to truly make a profit.
There are other threads on this forum that advocate providing estimates not based on hours but by project importance, how much $ the client has, what market you are working in, etc. which will enable you to make a profit while working more efficiently.