How long can my sims live at most before he or she dies?

Do you mean from birth to death or elder stage length?

From birth to death, the amount of days go like so:

• Short: 66+
  
• Normal: 132+
  
• Long: 528+

Elder life stage length goes like so:

• Short: 7
  
• Normal: 14
  
• Long: 56

I do have to mention that the lifespan variance also exists. The values I've provided are normal values. Each life stage could last a little longer or shorter for some sims. Though twins will have all the life stages last the same length for them.

In addition, Active and Long Lived traits will extend elder life stage. This will not reflect on elder's aging bar, but it will make your sim last longer when it reaches full.

How to setup long lifespan?

Aging settings can be accessed through Gameplay Settings.

For this, open the menu (three dots in the upper right screen) and choose Options. In the right, choose Gameplay. And in there, you can change the aging settings: set the length and even turn on or off aging (for played or unplayed households separately. It is also possible to have only the active household age).

This setting is different in each save. So you need to have loaded a save in order to change those settings.

True @JustinB113. I always have aging off, so my sims are perpetually young adults until I decide to age them up (which I haven't yet lol).

I play on Long, myself, with aging on. I've had to restart this save so many times, I'm almost immune to losing my favorite Sims.

As long as I want them to. Also, the don’t automatically age up. I always use CAS & some fun cheat code traits from one of my packs.??

i've always wondered how people play with aging on, i tend to get attached to my sims and dont want to lose them so every time i start a generations save i end up turning off aging only to realize generations is pointless without aging and so i go back to my main save

@ryouma17 I think it depends on the player. I'd get bored playing the same sims in the same lifestages for too long.

You can find lifespans in the gameplay options.
I play long lives with aging turned off. I use the cake to age up my Sims, but I may also plan to turn aging back on, and either had short or normal once my Sim population becomes too big.

I have one sim who is both active and long lived. He will probably live longer than his children.