Can someone tell me what is the exact or the estimated time on this screen time graph

Do you not see the big 2M above the graph?

Hint: that's the total amount of time. The "M" means minute

Hold your finger on the bar and a banner will appear telling you the exact duration.

Seriously you need to know to the second? I can only guess at it. A single line could simply indicate there was some usage in that period, maybe even as little as little as a millisecond. The total time for both periods is 2m. The single line is about 1/8 as tall as the bar next to it, which would make it 13 seconds, but 15 seconds sounds more normal (1/4 of a minute).My guess is 1ms to 15s. Unless you can click on the bar and have it tell you the duration or there is some way to see the results in tabular form vs a graph, that's probably the best you'll get.

On my iPhone, I don't see a bar at 2s but see the faintest of one at 15s. I tried it on my Mac, too, but it didn't seem to be working, it did not show any activity.

This does not appear to be a question about the Numbers app, which is the subject of this forum, but it looks like like a single line so it could be as little as a single second just to show there was some activity then. Why do you ask? Do you need an exact number?

More than zero, less than 15 minutes.