“She's only been gone one day.”
Your wife is gone on a business trip. She left yesterday. You have a young son who starts crying because he misses his mother. You say this, trying to get him to settle down and stop crying.
She's only been gone one day.
You describe an action as "only ___ for ___" when you want the time to sound short. This can be used to talk about the past, the future, ongoing actions, or imaginary actions:
I only slept 2 hours last night.
If you'd left by noon, you only would have had to wait for 20 minutes.
The word "for" is sometimes dropped in casual speech, as in the example at the top.
This is used to describe a continuous or repeated action that's still happening. The action can be something that happens continuously:
They've been arguing with each other.
Or it can be an action that is repeated over and over for a length of time:
I've been going to that grocery store for over 25 years.
You can tell long the action has been going on using "for (how long":
The beans have been boiling for about 10 minutes.
Sometimes "for" can be dropped, like in the example at top, in spoken English in casual settings.