4 seasons is the bare minimum you need to get from D2 to D1, in those 4 seasons you'll need at least 3 postseason appearances (2 of which are NT)
Example:
When Knight first opened I went from D3 to D2 in 1 season, then coached for 4 years at D2 going to the NT 3 times. Moved up to D1.
When Phelan first opened I went from D3 to D2 in 1 season, then coached at D2 for 4 years. I went in the PIT 3 times and the NT 1 once. I had to stay another year then moved up to D1 after a NT appearance.
In Crum now I went from D3 to D2 in 1 season, then coached at D2 for 4 years, I went PIT twice and NT once. I stayed another year and went to the NT again. Now moving up to D1.
In fact if you go back and look I don't believe any teams were able to move up after 3 D2 seasons. Even the coaches that won a NT.