Shouldn't take that long. On a heavily modded 1.3.7 cart, I found it on 30 pages on the admin side and 15 times on the public side.
Now this only counts the times it's in a language call, for example in /includes/languages/english/YOURTEMPLATEFOLDER/shopping_cart.php where it's on define('TABLE_HEADING_MODEL', 'Model');
You only have to change it in the language folders, make sure you are using the template override system correctly. Once you do that then everywhere on a page that it currently says "model" would then become "item", "thingie number" or whatever you want to call it.
A good php editor with find/replace ability with the "prompt to replace" feature turned on, take about 10 minutes.