There is no general safe distance, it's pretty much individual to each installation and depends mainly on the turntable, the loudspeakers, the turntable support and the room construction.
The way I would tell if it's OK is to put the stylus down on a stationary LP, select the phono source and turn the volume up to what would be a loud listening level and see if there's any feedback. With the volume control up as before, then gently tap the turntable or plinth and see if the tapping noise coming through the loudspeakers is clean, or if that sets off feedback. If you don't get any, then you're OK with the turntable in that position. If you do, then it'll have to be moved further away, or use other methods of isolation.
S