We always reply within the same half day the enquiry was received. I hate it when people do not reply to me and so I never ever ignore enquiries unless they are the obvious scams!
Sometimes when I am trying to pass business on to other local establishments - they do not reply and I am handing them business on a plate!
One thing to note is that sometimes replies we make to enquiries can end up in people's junk boxes. Sometimes my emailed replies do come back a couple of days later saying 'undeliverable'. So it is always worth checking your junk boxes to see if someone did actually reply - but it got binned!