Just what the question says. One could restrict the domain of nouns according to a certain frequency or top-X masculine nouns.
This may not be a proper scientific approach but it gives you a rough estimate on the occurence.
Of all 173782 nouns registered in Cannoo.net 3954 nouns belong to the en/en inflection class. Canoo.net.
By definition, all these "weak" nouns are masculine. However I have not found the fraction of masculine nouns from all nouns.