7 lives/years: maintainability and significance of the number 7 How valid or verifiable is the claim that a sotapanna leads no more than seven lives? Or a once returner returns once?

There is another claim that practicing earnestly for seven days or seven weeks or a maximum of seven years should be enough to attain nibbana. 

Whereas as far as I can see, nibbana doesn't seem too common at all. Certainly not the kind of numbers one would expect if it was possible in a few years.

Is there present day evidence for the latter? If there isn't then shouldn't it also lead me to wonder about the former?  

(Note: I'm not saying arahatship or lesser ariya states are impossible, just not very sure about the maintainability and significance of the number 7)

What is the learned opinion here? 

The Zen approach of nowhere to go, nowhere to arrive seems more commonsensical to the extent that there is nothing to disprove. On the flip side one can be really only wearing out one's cushion in the Zendo, and there'd be no way to verify one's attainment.
Ajahn Brahm addresses this in his book Mindfulness, Bliss, And Beyond. He notes that most people are not correctly practicing mindfulness such that it will allow them to attain stream entry. 

Although this is an alluring promise from the Buddha, understanding it requires a measure of brutal personal honesty: Are we in-fact practicing true, unwavering mindfulness during all waking moments of our lives for seven days straight? Such a task is actually quite difficult; even when we get a good, mindful groove going, it is exceedingly easy to lapse out of mindfulness and have to start again. 

I imagine that much of the meditation average folks do is primarily aimed at cultivating (i.e. learning, developing, and strengthening) mindfulness. It takes full mastery of mindfulness to be able to apply it long enough to break down our delusions to attain enlightenment.  

One final point Ajahn Brahm notes: depending on an individuals level of attainment (i.e. once-returner, arahant, etc) it may be very difficult to truly identify them. Traditionally, only the Buddha himself could verify these attainments. Thus, we should be careful in assuming how likely or unlikely occurrences of enlightenment are, though it is clear that even basic stream-entry requires serious work.