This is speculation, but I was telling this to Jonnie too, and he agreed with me.
I think it was around season 6. Reasons being: The birth of Nicky and Alex on Michelle's birthday was the 100th episode, which usually happens in season 4 or 5, with about 22 episodes a season.
The rest of that season and the next season was filled with Nicky and Alex as just babies who had no storylines.
The next season Nicky and Alex had a head full of hair, and that was their look when they went to Disney World. It was also with Danny's proposal to Vicky, and remember she leaves him in the next season b/c the "premise of the show was a single father raising his kids" as I remember reading in TVGuide. Vicky was not around for the final two seasons I believe.
So that is just me deducing. I know it's probably easy to figure out online, but I enjoyed piecing together the clues.
My guess would be season 6 or 7. Sorry not concrete, if anyone knows please post it b/c I'd like to see how accura
te or totally way off I am.