I've worked in retail (had to pay for college somehow), I've worked in IT, I've worked with the government, and I've worked with the military. In general, phone calls should be returned within 24 hours and emails within 48 hours. The problem here is that big ass companies that pay for an absurd number of customer service reps have changed the dynamic so much that people now expect return phone calls and emails within an hour or two. The reality is often something completely different:
Recently I got into a huge fight with HP's customer service. The tier 1 folks weren't able to help me out, so I asked to go to tier 2. The tier 1 folks could reply within minutes or even hours, because they didn't have anything to actually say. It was more of a repeat of what I'd said, and acknowledgement that they heard me. The tier 2 folks weren't much more helpful though, so I had to go to tier 3. The tier 2 person I talked to on chat. Having run some of those chat conversations from their side before, I understand that often you have three or four chats going on at the same time. With that said, there were often delays between replies of up to 10 minutes. For a conversation that (over the phone) might have taken 3 minutes, it took closer to an hour. With the tier 3 folks, I had to wait on the order of days to get replies. They were doing a lot of research into my issue, trying to find solutions, etc. This would mean that I could reasonably expect to get two emails a week back and forth. Five weeks later, I still wasn't happy with their service, so I stopped communications.
The point of me telling you that story is to elucidate the reality of customer service. Having been on both sides, I can assure that this was normal. Tier 1 service generally can't do a lot so the replies tend to be short and swift. Many places I've worked at in tier 1 IT expect a reply to be given to the customer (via phone, chat, or email) within a couple hours. The reply is often unhelpful in any situation that doesn't fall into the 60% majority of issues. Tier 2 replies tend to be issued within hours, simply because the issues tend to require a little more backend work and/or research to complete. Tier 2 people tend to hate escalating anything to tier 3, simply because by the time you get past tiers 1 and 2, they've covered about 90% of the likely scenarios. Tier 3 is where things get weird. They're dealing with situations that aren't common, and tend to have to do extensive research and back end work to resolve the issue. As a result, it's not at all uncommon to deal with tier 3 people over the course of several days.
Consider this: when have you dealt with a non-customer service person via email and gotten responses and answers within hours or even a day? Let's say you're an engineer, and you need some schematics. The person you contact might have them sitting right there, but probably won't. The person you contact will likely have to go find the schematics, verify that they are what you want, and find a way to get the schematics to you. Now consider that the person getting schematics for you has to wait on someone else to verify that the schematics are available, they've been verified, etc. Each extra link in that chain results in a delay.
This is why it is generally good business practice to give your recipient 48 hours to get back to you. Because the reality of dealing with people is that they are people.