I'm going to guess between human age-200.
In book 8 Ax states something like "if I live to be 200 years old I will still be known as Elfangor's brother." However, in my opinion, its impossible to tell whether he means that as a literal age or not. At that time in the series, Ax was not known for speaking non-literally, however the sentence lends itself well to a non-literal interpretation. It could go either way.
However in MM4, he states that the dome has enough energy to last 100 years, and that he could live out his lifein the dome. However, his tone is rather sarcastic and bitter in those parts of the novel, and he has begun to question his own sanity, so again its hard to say whether or not he is being literal. And I don't know if KAA wrote that or a ghost writer.
I would guess it is at least a little bit longer than a humans. We never get t a literal age on Alloran, but he was a full prince, I believe, in 1966. If we assumed he was super awesome and climbed the ranks of the military like mad, id guess he's about 20 based on the fact that Ax is an aristh at 13. At the time of him coming to earth that would put him at 50ish. And Ax never described him as an old man, he always acted like he was a prime andalite warrior specimen, so their lifespan has to be a bit longer than a human's.
edit: I reread HBC, in 1966 Alloran was NOT a prince, he was still a warrior. But he was made War Prince by the time he went to the Hork-Bajir planet.
