Not that often, actually... even though there is no shortage of them. It's been some time since I came across a rattlesnake, actually.
When we first bought the ranch, early 80's, I saw them often. Nearly every trip, sometimes more than one in a single day. They aren't as common as they were then. Never the less... if I went there with the intention of finding one I most likely could.
When I'm with my family, little kids along, I am very cautious about where we go, (I know well where they are most likely to be found), and I never stop reminding them to be watchful. No doubt they consider me quite annoying. When we hike we do so in single file...me leading... to 'sniff out', if you will, any snakes along the way. NOT fool proof, of course, but it works more often than not.
As you probably know... a rattlesnake bite for an adult 'can' be very bad but usually not with quick medical treatment. Rattlesnake bites for an adult are rarely fatal. With kids it's a different story, it can be very serious and serious very quickly. From my ranch it's about 1/2 hour to a hospital, (driving like a maniac, which I would be).