We cannot estimate anything, because we just have too little information about ET life.<p>We cannot even accurately estimate how long any given life form has existed.<p>For example, Earth is relatively young planet, and exists planets which are Billion years older.<p>And according to Turing theorem, we could only understand life form similarly developed or less developed than our.<p>So, if we will encounter similarly developed or less developed life form (younger than our), we could make decision what to do with it.<p>But if we will encounter much older life form, I think limit is just about few hundred years difference, we just will not understand them, may be even they will look like natural phenomena or super-natural as gods.<p>And when we will encounter less developed ETs, we cannot be sure, that nearby not hidden much more developed ETs, which could wipe us. So I think, safest for us will be to avoid any interaction and research them with remote sense technologies.