I usually interface with clients during normal business hours (9-5 PST, Monday-Friday). If circumstances require and prior arrangements are made, I am available outside of these hours.
Under most circumstances, I can give a rough estimate at no cost. When we begin working, I usually require half of a project's estimate up-front and the second half (plus additionals) upon delivery. I try to stick closely to my estimates and I always deliver on-time.
I usually prefer to communicate via email, but I'm also available for phone calls and Instant Message chat during normal business hours.