Oh...... Whoops! I made a very bad mistake Matt. I have forgotten that they divide and separate. I was thinking of something different. I was thinking that they grow more plants, coming from the roots. Some plants do that, but Dionaea just separate into smaller plants.
Oh, and if you say it's 'almost' impossible to tell how long they live for, can you still figure it out? Since you said that it will always be the same plant, even if it separates, can we just grow the plant until they all die? And look for the longest living one? But yes, that would take forever.