Last Saturday, a friend who was visiting my house commented that it looked like I hadn't tended to my lawn in a while. I realized she was right-- the weeds were growing pretty tall, and I hadn't cut the grass in weeks. I decided it was time to mow the lawn the next day. I went to the garage to find my lawnmower. I rolled the lawnmower over to the lawn, and then turned the lawnmower on. It started roaring, signaling that it was on and ready to cut grass. I mowed the grass in neat rows, starting on one side of my lawn and mowing all the way to the other side. I made sure not to miss any patches by going back and forth with the mowing machine. After I inspected my work, I decided I was happy with it. The grass looked much neater. I turned off the lawnmower and returned it to the garage. 
Question: When will they have to do it again?
When grass get taller again?