I think I had to wait for the next bus one direction, but I can't recall which. They run fairly frequently, and I wasn't in a hurry since I was not trying to catch a plane or anything. Pretty sure I only waited an hour or less, but that is entirely dependent on the route frequency. You can check the time tables on line and get an idea how far apart the buses are on your planned travel date.
And I just showed up at the terminal to buy my ticket. Did not purchase it in advance. My Spanish is good enough to ask one for ticket, by the window, please. And the word for return ticket is "ida y vuelta" e.g. coming and going. You can write it and the destination city on a piece of paper and give it to the clerk if you are unsure of your Spanish. My experience was that there was usually someone that spoke some English at most service establishments in Santiago. A little more challenging in Valpo and Vina but I managed with my half forgotten high school Spanish just fine.
Google translate is your friend.