The exact time or time frame in the past when an action took place (was completed) is not specified at all, unknown or simply not important:
Oh yes, believe me, I’ve been in this situation too.
As you can see, no point in time is shown here. And also time is not important here.
We can specify the entire past (but not a moment in the past):
I’ve seen that before. She has seen that in the past. They have met many times. I have visited Brazil several times (a few times). They have come here 3 times. We have never met. Have you ever met? She has always thought that this is likely to be a good start (this is a good example). I haven’t done it in my whole life. He hasn’t done it so far / yet, until now (in the entire past up to now).
We can point to the entire period (but not a moment in the period), usually we use in the last x years / days / etc.:
I haven’t seen such an interesting movie in the last 10 years
Another interesting use of Present Perfect. When we say
This (that, it) is the first (second, third etc.) time
It’s the second time I have done the same mistake.
“Times” in this context refer to an unspecified period including the entire past and up until now (so far), so the “entire past” is implicitly meant here. See more examples:
It is the second time I have done the same mistake so far.
This is the second time in my whole life I have done the same mistake.
In a nutshell (exceptions may apply)
We use Present Perfect when we have:
in the last few days, before, in the past, many / several / a few times, x times, never, ever, always, in my whole life, this is the first time (all of these belong to an unspecified period in the past, usually referred to as the entire past)
More on Present Perfect: