I frequently e-mail hotels and would say over 90% respond within a day. Interestingly, I find that small independent luxury hotels are often great about responding to e-mail. Two that come to mind are The Goring in London (which on several occasions sent a response within minutes, and this was not during business hours on their end!) and The Metropole in Venice.

OTOH, a couple of years ago I gave up trying to get a response from Hotel Eden, a supposedly 5* SPG property in Rome. I stayed there anyway and as it turned out, the service was not very good. Since then, my attitude has been "if you can't answer e-mails, I'm not staying there."