If you move a tree with its soil, it's considered replanted (so the rules of orla kick in) unless it can live off of the soil you're moving it with. (Source.) Clearly "if it can live" doesn't mean "if it can live for the amount of time it takes to move the tree", as, well, it does love that long, or else it's dead. On the other hand, no tree will live forever in some unchanging body of soil. So for how long a life does "it can live" mean?
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Thanks to Alex's comment elsewhere (which I only saw now), I discovered the Pische S'shuva, YD 294:13, who cites Parach Mate Aharon as saying the soil must last three years, and Shivas Tziyon as qualifying that that's only in eretz Yisrael: in chutz laaretz, he says, [where safek orla is permitted,] the soil must last "a few days".
The Pitchei Teshuva in Yoreh Deah 294 sk 13 quotes a number of opionions but rules that in Israel where we rule stringently about doubts relating to orla, there must be enough dirt to survive three whole years; in the Diaspora, however, where we rule leniently about such doubts, it only needs to be enough dirt to last a couple of days.