If you're cooking something that creates steam then yes you should leave your microwave door open for at least 30 minutes after use.
leaving the microwave door open after use helps allow the steam and moisture to escape the inside of the microwave which helps to prevent rust in the microwave.
It's also a good idea to wipe down the inside of the microwave after use as well.
If you're cooking something that doesn't steam then you don't need to leave the microwave door open unless you want too.
Most microwaves should last between 5 to 10 years.
And once the microwave begins rusting out it's a good idea to replace it.
If you get 5 to 10 years out of your microwave then you're doing good.
I've also had a few microwaves from Walmart fail me within a year or 2 but I did get a lot of use out of the microwave before it did actually fail.