Dates and timelines matter; not hours.
Not everyone agrees with me on this point, but I bring it up after hours of sometimes painful conversations with clients in many different fields.
Once upon a time, I worked with a client in California. They were quite picky, and the project was very complicated. Unfortunately, they weren't my only client.
I wrote a project proposal with a busy schedule in mind. The project was going to involve about 8 hours of effort. At the time, I billed $100/hour so I wrote up a contract for a $1000 budget - $800 for effort, and $200 contingency. The contract placed the project's finish date at two weeks later.
The client said no.
"If it's only an 8-hour project, why will it take two weeks?! We aren't paying you to be lazy. Are you only working what, an hour a day?"
Understandably, my client was slighted. They felt I was not giving them enough time - that I was focusing elsewhere and not placing priority on a project that, for their business, was mission-critical. They didn't have a problem with the budget - this was smaller than previous engagements. They didn't have a problem with the timeframe - two weeks was still a full month before their ship date.
They didn't really care about the number of hours I was working, but it confused the issue.
The Real World
You will almost always have more than one client - and every client will want to feel like they are your only, or at least most important, customer. In the real world, not every customer can be #1, so you have to prioritize work to keep things on track.
Your client doesn't want to hear that.
Put yourself in their shoes for a second - or think of the last time you went to the doctor's office. You are one of perhaps hundreds of clients. Hundreds of clients, each representing several dozen hours of labor.
How often do you have to wait to schedule an appointment? How long do you have to wait when you show up for said appointment? How much time does the doctor actually work on your case?
In many situations: the appointment is schedule a week or so in advance, you have to wait up to an hour to see the doctor when you do show up, and you only talk to the doctor for 10 or 15 minutes during the 2 hours you're at the office. You are not their only patient, and they have to move on to other obligations.
Doesn't that make you feel valued?
On the other hand, our job as service providers is to make our clients feel valued. We respond to emails quickly to provide a high level of service. We idle on Skype to provide support when things go awry. We sub-contract silently behind the scenes to catch up on an overdue project so the client doesn't slip a critical deadline.
A happy client is a client that feels valued - a client that feels like they're #1. A happy client cares how much a project will cost and when it will get done. A happy client trusts you to get the job done on-time and on-budget.
Stating how many hours of effort a project will take muddies the picture. It introduces more variables that confuse the issue. Even a happy client now questions your dedication to what they've always assumed was your number 1 priority.
"If this is an 8-hour project, maybe I can go somewhere else and have it done in a day. Two, tops. Eric won't have it done until two weeks from now ... is it really worth it to wait that long when I could ship two weeks earlier?"
Every time a client has to weigh a decision about your business relationship, you're at risk of them turning to your competition - someone who doesn't confuse the issue with extra variable that have no impact on the actual relationship.