I am 6'2, currently 87kg and have been training consistently for 18 month now. My 1 rep max for bench press is 115kg. What is a reasonable lifetime goal for max bench press for a natural lifter such as myself?
It is impossible to say.
Interindividual variation in well controlled strength training studies is already tremendous. Stop controlling variables, as in, just live your life, it is simply impossible to tell you what is a reasonable goal for you. 115 kg is already respectable.
But here’s the thing: chasing numbers is not a sustainable way to live your life. Hitting PRs is fun, but one day you’re going to hit your last one, no matter how great you are. What then? Are you going to quit lifting? You need to find a way to relate to lifting in a way that allows you to do it in perpetuity. Eric Helms and Omar Isuf have a great podcast about the tyranny of progress; it focuses on PED use, but the general principles about having a good relationship with lifting are universally applicable.