I don't expect them on call 24/7, but if they have logged on a quick reply positive or otherwise would suffice. If on holiday, they should take down their profile or have an automated reply message. Don't leave us hanging.
Some guy said it on here once. If they don't get back to a polite request for business within a working day (or two), then they don't deserve your custom. Same goes for any service. I now see it that way too, so don't chase anymore - especially given the notes system (for verrified SPs).