It's twenty years since I've seen her.

[5jj]

In this particular type of sentence, you may hear any of these:

It is twenty years since I last saw her.
It has been twenty years since I last saw her.

I haven't seen her for twenty years.

It is twenty years since I have seen her.
It has been twenty years since I have seen her.

They all have essentially the same meaning. I don't think I would say the last two, but there is no doubt in my mind that I have heard them.

 

[renzheng04]

Thank you all again.

 

[hoangkha]

If you said #1 in BrE, it would be slightly incorrect and would have to be changed to "I've seen her many times over the last twenty years". It would simply mean that you have met up with the same woman many times in the years since 1993.

If you said #2 in BrE, it would mean that you have been dating the same woman for the last twenty years (so since 1993).

In the red sentence, can OVER be replaced with DURING or IN, please?

 

[probus]

In the red sentence, can OVER be replaced with DURING or IN, please?

Yes.