(something) is on the decline

When something is getting smaller and smaller, or worse and worse, you can say that it is "on the decline".

Topics that you can use "on the decline" to talk about include:

• a person's health or athletic ability
• the economy
• crime statistics

I'm getting older now and my health is on the decline.

The simple truth is that this once-great nation is on the decline.

The murder rate is on the decline, and has been for the last decade.

quite a while

"Quite a while" means "a pretty long time". It's not a really long time, but it's also not a short length of time.

Exactly how long "quite a while" means depends on the situation. It can mean minutes, days, or even millions of years.

A: How long did you have to wait for?

B: Quite a while.

I haven't seen her in quite a while. What's she been up to?

(something) has been (happening) for (some length of time)

This is how to talk about an action that started in the past, and is still happening now:

I've known him for over 10 years.

You can also use "since":

We've been friends since 1990.