So I did some informal testing, with probably a large margin of error, I have some pictographical results.
I picked Blitzcrank, because I own him(it?) and he has a base 300 ms. 8 Seconds of walking down the bottom lane put me a little ways behind of the inhibitor.
Purchasing boots of speed boosted him up to 350 ms, and 8 seconds brought him past the inhibitor, put not yet past the inhibitor turret.
I attempted to record results for 406 and 455 (like I said, it's hard to get the higher numbers precisely) but botched the results, and in each case I was only about the same spot as 350 ms.
Finally, I bought Phantom Dancer and Boots 5, hit ghost (un-talented, sadly) and activated rank-5 overdrive to see how far I could get when speed was pushed to egregious levels. Since Overdrive lasts for 8 seconds, it made for a far more accurate timestop. I made it here.
So we can safely conclude that, unless you're backed in to your own base, it is always faster to recall.
Now if you'll excuse me, I'm going to go avenge my horrendous 6 - 4
previous test game. I will get Ashe Bot this time. grumble grumble
Best Answer
The answer to this is really simple, ask yourself the following question :
"Do I get a benefit from interrupting him ?"
- if yes do it !
- else don't !
For your lane opponent, if he just hard pushed the lane and you see him back, prevent him from doing it ! Because if he does recall, he'll barely lose any cs so make him lose time for you to prepare the push for when he is gone.
Other case, if he wants to recall because he is out of sustain (pots, flask, low health) or because he is behind in items and need to shop, PREVENT THE RECALL. You have an advantage on the lane, don't let it go.
If he is too far, and you need to overextend or lose cs, ask yourself the same question again, does it still gives you a benefit to do it ?
And for the jungler, if he is backing on a ward he doesn't know about, it might not be a good thing to prevent the recall and give the information that it is warded there, because when he comes back to your lane, he will know what to avoid. But if he is doing a stupid back in a visible place, go for it.