To answer your question looking at an actuarial life table is useful. Given that the expected remaining life span of an 81 year old US male is 7.43 years from a 2021 table for Social Security, I’d say he likely would complete his second term as President.
Biden, unlike many others, will have access to top notch health care that increases his chances of not dying on the job, so to speak.
Is this answer satisfactory to you? I even gave you a credible source for what I based my answer upon, that is, I did not pull it out from my ass.
Actuarial Life Table
www.ssa.gov