Same day is preferable, but within 24 hours is necessary. If you are actively shopping, and others are responding, you can't wait a week to hear back from someone. Especially in a busy market. There is one vendor who had nice stones, but the 3 days to week+ response time took them out of the running since I kept moving forward, with or without them.
I used to required that my team responded within 24 hours to every inquiry we got. It doesn't matter if it is diamonds or equipment. When people are trying to purchase something you need to be responsive or you will lose their interest. Basic sales/marketing 101.