The tincture will be good for years. They will lose potency over time as THC is not stable forever and will degrade to other cannabinoids, mainly CBN. The container you use and your storage (cold, dark, and dry) will determine how many years your tincture is good for. The strain you use may also have an effect on things, but you would most likely have to undergo years of tincture study to really understand how long a tincture will last for.
I have heard of individuals that just keep a tincture going indefinitely, slowly adding more stems and leaves to a mason jar of whiskey.
It can go bad if stored improperly, but quality of tinctures also depend on the extraction method. Solvents may have been used to purify your tincture and prolong its shelf life. Luckily, there are a lot of resources on our site on this subject.
To learn more, check out this article!
Also, here are some best practices for preserving your cannabis products:
No need for alarm, but I also recall there is an article which can help you spot dangerous mold in flowers!