As Flars said, there's no specific formula, but a rule of thumb is that after 30 minutes, so much of the aroma is lost that you might as well just consider it bittering. This is of course only a rule of thumb and there are a lot of variables including the variety of hops and even their freshness. But it's why, for example, 45 minute additions are uncommon. The most common schedules are something like 60, 20, 10, 5 and flameout or variations on that, such as 60 & 5. The later the hops are added, the less the alpha acids are isomerized and the more of the aromatic oils are preserved.
I've been making some good beers by hopbursting. That's where all of your hops are added at 30 minutes or less. Of course it takes more hops to get the same IBUs, but that's the point. E.g., if I had a beer that used 1 oz of 8% hops at 60 minutes and 1 oz at 5 minutes, I get 36 IBUs; if I move the "bittering" addition to 15 minutes, I'd use 2 oz for the same 36 IBUs. But since those hops aren't being boiled for as long, much more of the flavor and aroma is preserved. It's kind of blurring the idea of "bittering" vs "aroma" hops.