How long will homemade marinara sauce keep in the fridge?
Assuming that you've limited the original sources of contamination and plan to simmer the sauce for at least a little while before serving, you should be good for at least a week. Tomatoes tend to be fairly acidic, so a marinara sauce is a less-than-ideal growth medium for bacteria.
But the key here is the assumption that you have a minimal number of bugs in the sauce to start with. Simmer it for at least a few minutes, then put it into a storage container (or lid up the pot) while the sauce is still hot. If you let the sauce cool to room temperature and then use a dirty spoon to scoop it into a storage container, all bets are off.