I've been driving with my pads worn out on the rear for a little while now, due to lack of time to get them changed etc.

Anyways i stopped driving it for the most part till my pads come and i get em changed.

But a lot of grinding has happened and my rear rotors have worn down a bit. a few mm on the top of the rotor.

is that enough for me to get new rotors? or how do u know when u need to buy new ones?

if i dont have to buy new ones id rather not at this time.

The way you find out for sure is by taking them to a machine shop, have them cut and measured. If they are still within specs, you are good to go.