Learn English – In two days’ time = within two days

As @DavidRicherby said, just "rainfall" would be more natural than "rainfall activity."

Sentence 1 – in

The rainfall has gone down in the past two days.

This means that, at some point in the previous two days, the volume of rainfall has decreased. There is no indication of whether it decreased gradually over the two days or not.

Sentence 2 – over

The rainfall has gone down over the past two days.

This sentence has at least two meanings that I can see:

1. That the amount of rain that is falling has, over the past two days, decreased; or

2. That, in comparison to the volume of rain that fell in some other time frame, the volume of rain that fell over the past two days was less.

Here's an example for the second meaning:

On Monday and Tuesday we measured 15mm of rainfall. On Wednesday and Thursday we measured 10mm of rainfall. Today is Friday, so the rainfall has gone down over the past two days.

Sentence 3 – for

The rainfall has gone down for the past two days.

This has a similar meaning, but to me it has different connotations. It means that the volume of rainfall has gradually decreased, but it also implies that multiple measurements were made over the course of the two days and at each measurement the volume had decreased.

Sentence 4 – during

The rainfall has gone down during the past two days.

This sentence doesn't sound natural to me. It would certainly be understood, but it doesn't sound natural. However, this may just be a dialect difference between British and American English. If we use Google Ngram Viewer, we can see that "during the past two" used to be significantly more common in American English than it was in British English:

American English:

British English:

Using over is the safe option, since in both cases during is declining in usage. However, it does appear that during is correct.

It's pretty unlikely that your boss or teacher will tell you something generic like "you have two days" without also telling you the day of the week or date it's due.

But, if that were the case, you should ask for more specific information because there's no one correct answer.

Let's start with a couple of work examples.

1. It's 9 am Monday. You're given a project and told "you have two days to complete this project".

This generally means you have two work days... so that means you have all day Monday and all day Tuesday. The project is due end of day Tuesday... usually meaning whenever you go home for the day (5 pm). You might be able to say it's due first thing Wednesday morning as that's effectively the same thing.

2. It's 5 pm Monday and your boss gives you a project and tells you "you have two days to complete this project".

In this case, you're about to go home for the day, so this is your assignment for the next two days... Tuesday and Wednesday. The project is due end of business on Wednesday (or first thing Thursday morning).

In either example, the result is the same... two full work days to do the task.

Similarly, in a school situation, if the teacher gives you two days, that usually means you have two days from today... so if it's Monday, you have until your class period on Wednesday, although some teachers who allow digital submissions of work will give until midnight - this should be spelled out specifically by your teacher, though; don't expect it to be standard.

So, the "safe" bet is to take today's time and add 48 hours to it and assume that's when it's due.