Signs that your microwave is going bad are it takes longer to heat or cook food, the microwave is not heating up fast enough, the microwave stays on when the door is closed, the control pad is not working properly, you smell smoke smells, or the microwave makes burning smells.
Or if the microwave is 10 years old or older then it may be ready to go out on you and you should prepare to buy a new microwave.
A microwave should last between 5 to 10 years when used and maintained properly.;
However in some cases a cheap brand of microwave may only last a year depending on how often you use it.
I normally get around 5 years and sometimes 10 years out of a microwave.
My last microwave that I bought back in 2010 lasted for 10 years before it gave out on me.
It was a Walmart brand microwave and I got my money's worth out of it many times over.
Most often the control panel will go out before the actual microwave will go out and stop heating.
If you get 5 to 10 years out of your cheap microwave then you're doing good.