Romans 11:25 For I do not desire, brethren, that you should be ignorant of this mystery,
lest you should be wise in your own opinion, that blindness in part has happened to Israel
until the fullness of the Gentiles has come in.
Is there a quantitative means of determining the conditions
under which "the fullness of the Gentiles have come in".
It depends on how you understand ethnos in this statement in Rom 11:25. Is it to be understood as gentiles, nations, races, tribes, or heathen? Paul has given some clues in the chapter-passages leading up to this statement. There are seven mountains, there are seven nations greater and mightier than Yisrael, and there are seven kings. So then, if you read ethnos here as heathen then the fullness of the heathen will be when the transgressors have come to the full. Beware especially of the Girgashi, they are Legion.
Deuteronomy 7:1-6 KJV
1 When the LORD thy God shall bring thee into the land whither thou goest to possess it, and hath cast out many nations before thee, the Hittites, and the Girgashites, and the Amorites, and the Canaanites, and the Perizzites, and the Hivites, and the Jebusites, seven nations greater and mightier than thou;
2 And when the LORD thy God shall deliver them before thee; thou shalt smite them, and utterly destroy them; thou shalt make no covenant with them, nor shew mercy unto them:
3 Neither shalt thou make marriages with them; thy daughter thou shalt not give unto his son, nor his daughter shalt thou take unto thy son.
4 For they will turn away thy son from following me, that they may serve other gods: so will the anger of the LORD be kindled against you, and destroy thee suddenly.
5 But thus shall ye deal with them; ye shall destroy their altars, and break down their images, and cut down their groves, and burn their graven images with fire.
6 For thou art an holy people unto the LORD thy God: the LORD thy God hath chosen thee to be a special people unto himself, above all people that are upon the face of the earth.
We are to make no marriages with them: for it would be no different than as in the days of Noah. We are to destroy their altars, and break down their images, and cut down their groves, and burn their graven images with fire, and make no covenants with them. We are to show them no mercy and cut them off before they choke the seed of the Logos from the soil of the heart, (we wrestle not against flesh and blood: you cannot see this genos of heathen with the eyes of the flesh, but they may indeed creep in alongside unaware to us as we sit down to our love-feasts in the Word: as brute beasts, and clouds without water driven by the wind, as autumn trees having gone past the time, bearing no fruit, etc., etc.).
So if you read
ethnos in Rom 11:25 as heathen, their number is seven, and yet they are legion, and their distinct characteristics are expounded in various places throughout the scripture so that we may recognize them and their evil.