What is the difference between
It's the last time I've explained this to you.
and
It's the last time I'm explaining this to you.
I mean, this is the situation where I'm explaining something to someone right now. Can I use "the last time" with the Present Perfect or only "the first time/second/third..." when I talk about present actions?
The main difference between them is that one of them is wrong; the other is grammatical, but not quite idiomatic.
It's the last time I've explained this to you.
This is incorrect, because "it's the last time" is referring to the present, while "I've explained this" is past tense.
It's the last time I'm explaining this to you.
This version is fine and might be something that somebody would say, speaking extemporaneously. However, if I was to write this, it would be:
It's the last time I explain this to you.
This is the last time I will explain this to you.
generally the will auxiliary is used, even if the thing is happening in the present. The will explain will occur right after the sentence is uttered. In this case, the will explain means: that I have the intention of explaining this to you.
You can also use the present progressive.
This is the last time I'm explaining this to you. [I am doing that right this second.] or
This is the last time I'm going to explain this to you. [future right at hand]
