What happens to it if you don’t eat it within 6 weeks? Can it last much longer?? Also, can you use it with similar fruit, but fresh?

This was a trend back in the seventies, derived from a German tradition I believe called “Rumtopf” and my dear sister did this in a crock and kept it going for years by adding fruit and topping with alcohol until she gave the whole family food poisoning….

There are traditional recipes for fresh fruit in brandy (cherries, peaches and the like) that last at least from summer harvest to Christmas (so minimum six months). High in sugar and brandy, to safely preserve them. I've made the cherries and they're terrific!
See sites like the Spruce Eats, allrecipes, Ball Canning for recipes and proportions.

The recipe header says that "the fruit is best consumed within a few weeks, before the fruit's sugars begin turning the booze to syrup." You can't do this with fresh fruit, as the dried fruit soaks up the alcohol, which plumps it and makes it what it is. I think with fresh fruit, you'd just end up with infused alcohol and maybe moldy fruit.