If your reply is simply "I'm overloaded with work [... etc]", should it really take two days to send out?
As a (part-time) freelancer, I don't have the problem of being overloaded with freelance work (not yet!), but when coupled with all of my other responsibilities, it does get demanding.
I will say though that when I'm on the other end and hire contractors, if I leave a message to say I'm interested in having work done, I typically expect a call back within 1-2 business days. 3 is pushing it, depending on the service. If that doesn't happen, I assume they are
A) Unprofessional or
B) Too busy to provide me quality service at this time.
Time will tell if it's A or B. Eventually, if they do return my call, I still may ask for a quote from them if the work is still available, and may even go with them for this project or a future project. I understand some companies have busy seasons (landscape companies for example), and being busy myself I understand if small shops are overloaded at times.
Your response time would ring in right on the edge in my book. It would be a -1 though just to hear that you'd have to follow up with me next week. If the response is a 2-3 day period, I would expect the consult pretty much at that point in time, and not have to wait a business week just for a quote.
As a remedy, perhaps set some time aside during the day for following up with prospective clients. A 24 hour response time to touch base with them to schedule a future consult could go a long ways, and if everyone has the time right then and there you could do the consult then. The key word here (obviously) is schedule, a prompt reply to schedule a future meeting can keep your foot in that door, because rest assured that client is (at least should be) contacting other companies/freelancers that provide the same service, and don't want to wait a couple of days just for you to say your too busy and will call have time back next week. It's a microwave world now, coal burning ovens are a thing of the past. Calling and scheduling a meeting with them would at least hold them off from committing to another company until they've heard your offer.
Of course, it could go the other way; if your reputation precedes you, they may even want to wait for you to have the time to do their work, because they've heard or know just what kind of work you produce.
That being said, if you're too busy to provide quality work, you're too busy to take on the client at this point in time. It would hurt to turn down a client, but you should at least follow up with them quickly, perhaps the time frame for completing the project does line up with your future schedule, and if it doesn't then you've made a great impression and they may contact you for further work (nothing wrong with being a backup).