If you're an independent consultant, the amount of work you get from different clients may vary widely over time. You might go through long intervals in which you only work for one client, stretching the definition of "independent" (maybe even too much for the IRS). Then at other times, you might find yourself working for three or more clients at the same time. I even had one month for which I sent out invoices to seven different companies. During those periods, how do you manage your time to adequately satisfy all those people who are eagerly awaiting the fruits of your genius?
Naturally, you have to start by evaluating their individual needs. Some clients will want to get a specific minimum number of hours from you each month. They may even have requirements for the specific hours during which you must be available to them -- but hopefully not. Even though a fixed number of hours slips easily into a regular routine, you'd like to have the flexibility to move those hours around if another client has an emergency, or if the usual starting time comes along but you're productively working on another problem. Likewise, if you're suddenly seized by a fit of inspiration during "off" hours, it's nice to be able to put in that time impromptu and still get paid for it.
Then you have the clients who may call you once a month for some small project. Maybe it's just fifteen minutes of advice, or whipping up a little utility that takes a couple of hours. These clients are pretty easy to deal with, because you can almost always fit them in.
The hardest clients to manage are those that you don't hear from for months or even years at a time, and then they suddenly call you because they desperately need your help on a huge project that could easily consume all of your time. You already know that once that project is done, you won't hear from them again for quite a while. Do you push your other clients aside to make room for this one, and then hope to make amends afterwards? Or do you tell them you're just too busy, and offer to help them find someone else?
First, step back for a moment and think. How much time could you really give them, and still keep your other commitments? If the answer is zero, you must say so -- unless you're willing to lose those other clients. Otherwise, tell your prospect exactly what they can expect from you. If they say, "But we really need you to be dedicated to this project full time!" don't budge. Explain that you've made other commitments -- even if they moan and groan, they'll infer from your loyalty to your other clients that you wouldn't bail on them, either. Then, if they can accept the time that you have available, take the project. It's all about setting the right expectations.
When evaluating your available time, don't forget about reserving some time for self-improvement. Add to that all the time you spend doing not-specifically-work stuff like reading and responding to e-mail and feeds, billing, collections, paying bills, managing your own network, blogging (unpaid, that is), chatting, gaming, getting coffee, and going to the bathroom. Unless you're willing to give up, farm out, or cut back on some of those activities, you have to be honest with yourself about how much time you actually burn on them.