Alexander died IOTL in 1881, at the age of 62. Let's say that he avoids this and all subsequent attempts on his life and lives to see a natural death. How long does he live and how does his longer reign affect Russia and the world?
Taking a quick look at a list of the Russian Emperors possibly not that much longer. His father and the three proceeding Tsars all dies at a younger age than he was, likewise the following one. Of his brothers one had a shorter lifespan, another only lived for two years more than he did, but his youngest brother lived fifteen years more than he did. Now of course this takes absolutely no account of personal circumstance, I've got no idea how healthy or vigorous Alexander was at the time of his death, so you could decide to take it in either way. An extra five or ten years maybe?