How to replace polenta in Mexican Polenta Casserole recipe
Corn grits are not the best possible substitute for polenta:


Polenta is the Italian name for corn meal
Instant polenta is basically polenta that has been cooked, then dried and ground; it is precooked, and and pre-gelatinized, and so does not require long cooking
Precooked polenta is available, in a tube or tub, which is literally a prepared, cooked polenta.  
Grits are a ground corn product, where the corn has been treated with lye (it is made from hominy)


A better substitute for uncooked polenta would be plain corn meal (1:1); for precooked polenta would be cornmeal that has been cooked into a thick porridge (1:1).

If you do substitute hominy type grits for plain polenta, your result will be somewhat different, but you can try 1:1, by volume or weight, either way.

Update:  evidently some processors label their product "Grits" as well as "Traditional Italian Polenta".  While I cannot endorese misusing the word grits, in this case, then, they are the same product so it isn't really a substitution, and 1:1 is the proper use.