How to output a Picture Box at a random time?

I want a picture box to spawn at the bottom of the form at a random time interval within the program. I've got this piece of code whih randomises the location of the picture box but I'm unsure on how to do it between a certain time range and have it random:

Random rand = new Random();
picturebox.Location.X = new Point(rand.Next(0,this.Widht));
pictureBox.Location.Y = new Point(rand.Next(0,this.Height));