depends on where you are sailing, the weather and the type of dinghy.
I have a rigid dinghy and I keep it about 15-25ft back.. ive seen some being towed at 100ft.... some say to tow them on the backside of the stern wave and others say to tow them on the face of the stern wave... in stronger weather, ive tried towing it from my leeward cleat, and this puts it in the waves, and when towed from the windward cleat its in the wind... so i just shorten the bridal tow line and keep it closer behind the boat.
in big waves, there is no safe way to tow a dinghy... if you attempt to tow it close, you can over stress the tow line due to the jerking of the line when the boat pitches, and too far off it will be vulnerable to capsize or swamping.