Homemade nacho cheese will last around 4 to 5 days in the refrigerator as long as you keep it properly sealed up in the refrigerator.
Store bought nacho cheese seems to last longer than homemade nacho cheese for some reason.
For store bought cheese it lasts around 4 to 5 weeks as long as it's constantly refrigerated when not using it and kept properly sealed.
If you buy store bought canned nacho cheese and never open it then it can last for a year and longer.
So as long as you use the store bought nacho cheese within a month it will be okay in the refrigerator and if it's the homemade nacho cheese it will be okay for around 4 days before it begins to go bad.
You could freeze the nacho cheese and make it last longer and then just thaw it out when ready.
When frozen the nacho cheese should last for 6 months to a year.