You should replace your phone as often as every 2 years to every 5 years.
How often you will need to replace your phone will also depend on how well the phone is cared for and it's model and make and how well it is made and working.
Most phones are made to last for around 3 years before they show any signs of degradation, such as slow performance or a weak battery.
Although some higher end phones and well maintained phones may last longer.
Phones that are well cared for and protected such as from water damage and drops can last much longer than phones that are mistreated and not cared for properly.
And if the phone struggles with performances, runs slow, freezes up or has a battery that is not staying charged or draining faster than normal then it's a sign that you should replace your phone.
And if the phone no longer gets the essential software and security updates, it's also a strong indication that it's time to upgrade to a new phone.
And after around 5 years, the core components like the battery and processor might have degraded to the point in which a replacement is needed to ensure data security and functionality .
If the battery no longer holds a charge for a full day or the phone gets consistently slow, lags or crashes then it's time to get a new phone.