I've had some bad experiences with hair dressers getting too scissor happy so I don't go to one unless I really need a trim and my ends are pretty bad. I also only go if they will cut my hair dry and when it is straight so I can see exactly how much they are cutting.
Usually I trim my ends myself because they are rather healthy and I only do small trims. I trim whenever I see some split ends but usually I just dust my ends and that is enough. I don't have a set time schedule to cut my hair I don't believe in that I think you should cut your hair when you want to or need to.