I would think that ST's lifespan would depend on how big of a hit SW:GE ends up being. If it's only a giant hit, Disney might be able to get rid of ST relatively early.
If it's a Potter-sized hit (I don't use that lightly) that fundamentally reshapes which parks people go to in Orlando, then ST could be around even longer than 2-3 years. I could easily forsee a world where Disney is forced to throw together 1-2 brand new (not replacing anything) , decently large rides in DHS within a year or two, so that other older rides could be taken down for refurbishment without throwing off the entire park. Only at that point could Star Tours be taken out of commission.