Maximum or minimum?
Max...as high as you can get it, using the best coax you can afford. Minimum...well it depends on what freqs you wanna hear. The higher the freq, the more line of soght it is, so you kinda need to 'see' your target with your antenna. Not quite, but a good rule of thumb. This all depends on you nearby terrain, etc.
Not much help I know, but there is no magic bullet.