It totally depends upon two to three factors ,
CPU: If you are using a high end CPU which produces a lot of heat or a overclockable one like Ryzen CPUs or intel ‘K’ series CPU (and you overclock them) , you need to change the thermal paste regularly around 3 to 4 months , as it get burnt due to excess of heat , to maintain the clock speed of CPU you need good cooling in which thermal paste plays a very important role.
Work Load: If you pour extreme loads on your CPU for longer duration of time like playing intensive games non stop for more than 12 hours or CPU rendering stuff for overnight , the thermal paste in this case also looses its conductivity so its better to change the thermal paste once in 6 months in this criteria.
Your RIG: If you have a super compact PC without sufficient air flow a very good example is laptops than definitely you require to change the paste before summer seasons or a year.
In our country its best to change the thermal paste before the starting of summers as it gets very very hot.
The stock Thermal paste is always not of that quality compared to third party ones like thermal grizzly … it almost have a heat conductivity of more than 12W/mk and much more heat tolerance than stock ones.
If you dont have any type of usage mentioned above than also you should get your PC cleaned or serviced once in a year bcoz the overall dirt accumulation over a year gets humid and can result in bad temperatures and also degrades life of your PC components.