A question about effects tails

1. Make it as long as you want
2. Make it as long as space will allow (i.e. until the tape runs out)
3. Just let it go. Even if you can't hear it your dog still can so let it go, and go, and go...
 
I'm no expert but I usually end it unnaturally with a fade-out where it makes sense to, but still sounds relatively natural, so that I don't have the above situation.
 
Point being, even the dog's tail ends at some point, especially if he's a Pointer.

 
Too much tail just gets in the way.

 
As does too much nail.

It is a good question. I have been known to produce ambient tracks myself and in these situations there can be a lot of slow release envelopes present.  Including long reverb tails.  I tend to solo the tracks and just turn the system up and use your ears little more.  Listening until you reach the point of not hearing it any more.  And if you compared it to the figure of -60 db as mentioned here, you will find that figure will be higher in the real world e.g. a sound becomes hard to hear at say -40 dB.  Or even -30 dB.  Depends on the sound and its contents too. Sounds that contain a lot of frequencies e.g. more noise like will become hard to hear even sooner.  
 
I say just always render well beyond where you think a track or sound may end.  It can catch you out sometimes and you find yourself with a track that gets cut off too early or abruptly.  Then you have to fake the fades there.  Or try to extend a sound beyond its original end.  By cutting pasting and fading etc.. At least we can do it now.  Reverbs can help in situations like this though.
 
In the old days I was dealing with a lot with how tracks faded out on the tape multitrack into the tape noise. Noise reduction here lowered the tape hiss a lot.  Some hardware synths are noisy in terms of their quiescent noise level. e.g. early Kurzweil K2000's, Oberhiem's.  Not all those amazing analog synths of the day were quiet.  At least today our virtual instruments are, and we can control how they fade out with slow sounds.  Keeping our digital tracks these days super quiet also has made a big difference. 
 
So the fades on individual tracks right at the end of your music can be set up nice.  But for the best signal to noise ratio, the final fade should be done in an editing program and apply the final fade out there over the whole mix. That will ensure any noise is also being faded away. 

just make sure you have 'something' in your tracks, that allows the end of the song to play past the point of the reverb decay.
otherwise, you will hear the effect chop off, even super low levels, casual listener might not be aware of it, but will know it sounds weird.
 
i've even put a volume envelope on a reverb buss before, with a long reverb with long tail, and used the envelope to draw the volume down before the tail actually finished,
just to control the effect to blend it out with the rest of the tracks.