Then best-of-seven afterwards.

If you use this last season as an example with the first round of the playoffs starting Saturday April 16th, they played 4 games each on Saturday and Sunday, with 3 games each on Monday through Friday. In a best-of-3 round with 16 teams, you would have a minimum of 16 games (all series are a 2-game sweep) and a maximum of 24 games (all series are 2-1). That would mean the first round ends on Wednesday (April 20) at the earliest and Saturday (April 23) at the latest.

If the 2nd round was best of 5, then the 2nd round ends Wednesday, April 27 at the earliest (assuming all 1st round went 2-0, all 2nd round went 3-0). On the long end (assuming all 1st round went 2-1, all 2nd round went 3-2) it would end Wednesday, May 4. The "average" 2nd round would end on Sunday, May 1st.

Compare that to this season where the 1st round started on on April 16th and ended on April 29th (though not a single series stretched to 7 games). Basically, adding an extra round but shortening those rounds to best-of-3 and best-of-5 would only add 3 days at most onto the season (which would be more than balanced out by reducing the regular season by a small handful of games).