The Simpsons

well its kind of coplicated.
the simpsons exist in whats called a frozen timeline. in other words while time still progresses and the world around the people grow and advance, the people them selves don’t.
like in the early episodes the simpsons would have the technology from that time era yet the newer episodes have more mordon technolgy (cellphones, better computers, obarama as president, ect)
so to answer your question around 20 years, the same lengh as the shows has been airing however in the acually aging and passing of the characters no years.

It’s been on-air for 20 years. However, the show is in a perpetual now, so time doesn’t pass nor is the current year stated. No time has passed at all during the run, and it’s best not to take the timeline too seriously.