Which of the following sentences is incorrect?

Period of time (কত সময় ধরে) বুঝাতে for বসে।
- She was waiting for four hours.
Not: She was waiting since four hours.

আর Point of time (কোন সময় থেকে শুরু) বুঝাতে since বসে।
- I’ve been swimming since I was three years old.
- I’ve been swimming from the age of three.
Not: I’ve been swimming from I was three years old.

We use since, not ago, after ‘it’s a long time’ when we refer back to a point in time:
- It’s a long time since your last letter.

Source: Cambridge Dictionary