Going for a different club or to get another ball is specifically sanctioned as is being said. Chances are you will have said what you intend to do, but your actions will clearly show your intention. Going back to your bag , putting your club away and hesitating because you think maybe you should have played a PB and then going back on to the tee to do so would, in my view also be ok. You are, in terms of the rule, saving time and have not headed off "in search of your ball."
Putting your club back in the bag, and setting off forwards expresses a different intention: you are clearly going forward in search of your ball. So how far forward could you be allowed and still meet the criterion of saving time? Not far at all, I reckon. What we need to consider is the point at which the time saved by playing a PB should your original ball be lost, is overtaken by the time lost going forward a bit, then back to play a PB, then forward again should your original ball be found.
The rationale is, I think, that playing a PB immediately takes up an insignificant amount of extra time should you find your original ball and obviously saves a great deal if you don't. Coming back from even as short a distance as, say, 30 yards takes up an appreciable few minutes. Because there is a good chance (finding your original) that this time is wasted, the sole purpose of saving time would not be met. It has to be pretty tight: the wording is not "the sole of purpose of possibly saving time".
Deciding 50 yards up the fairway and going back would be clearly be out of order. That's 50 yards of going forward to search for your ball, with a goodly chance that the time spent on the 100 yard walk to play a PB is wasted because your original is found.
Personally, I wouldn't be happy with more than around 20 yards - in effect clearing the front of the tee or clearly and purposefully walking on with your club back in the bag from a shot through the green.
As you are using your own standard of acceptable yardage here, plus there is no stipulated distance then i'd like to think we would ALL use common sense! As the sole purpose of the rule is to save time then technically you can walk as far as the point of no return!
However, in the real world I can't see anyone falling foul from playing partners over this rule.