Rule for present perfect continuous

Question

Consider the following sentence:

• Have you been watering the plants?

Is the above sentence grammatically correct? Or should it be something like:

• Have you been watering the plants for 5 minutes?

i.e. when you see the use of 'have been' with present participle form you also use the time of start.

Answer 1

You can certainly use the present perfect continuous without an expression of time. In such a case you are interested only in the activity, not in its starting point or duration. For example:

• Your shoes are wet. Have you been watering the plants?

If you want to emphasise when you started the activity, you need to use since:

• I've been watering the plants since 5 o'clock.

And if you want to emphasise the duration, you need to use for:

• I've been watering the plants for 5 minutes.

The specific question Have you been watering the plants for 5 minutes? is grammatical but unlikely. One possible scenario is that your wife has asked you to water the plants for 3 minutes only and suspects you might have exceeded the time limit she set.