[noun (period of time)] + というもの(は)、[sentence] この / ここ + [noun (period of time)] + というもの(は)、[sentence]
というもの after a stretch of time means "for the whole of": the state that follows lasted the entire period, and the speaker feels its length. It goes straight after the time expression – この一年, ここ数日, 三か月 – and は can follow it. What comes after is usually a continuing state with ている or a negative.
The pattern belongs to writing and formal narration: essays, novels, speeches. For "ever since something happened", the related form is てからというもの, which has its own point.