The correct amount of insulin is the amount you need to keep your level stable and in range.
This varies hugely by person. Smaller people, typically, need less. More active people will need less. People experiencing insulin resistance will need more. Then there are other variables such as needing more insulin if you are unwell or had a bad night's sleep or taking certain medication or ate more carbs or ...
Unless you are experiencing hypos, you are not taking too much insulin ... for you.
It makes no sense to compare yourself to others - we need as much as we need.
What calculation did you use to take 120 units a day, is it based on your weight (see used often in US) or advised by your health team?
I take 10 units of Lantus at 6pm each day and carb count taking 1 unit of rapid acting NovaRapid per 10g of carbs, so varies but average of 30 units per day.
So a pen lasts about a week or more. We are of course all different.