The Tribulation has been in effect since the giving of the Revelation to John.
It won't end until Christ Jesus returns.
The "3 1/2" years theory was invented by the Jesuit,
Cardinal Bellarmine,
who died in 1621.
His theory placed the arrival of the Antichrist in a far,
unknown,
distant future,
because the Reformers
(Catholics who wanted to reform the Catholic church to conform to the real Gospel)
were saying the Papacy was the Antichrist.