According to ESPN's Adam Schefter, people around the league expect the 49ers to decide Alex Smith's fate "well before" April 1.
On that date, Smith's $7.5 million salary becomes guaranteed if he's on the 49ers' offseason roster. Per Schefter, the 49ers will owe Smith "nothing" if he's traded before March 14. If he's released before March 14, the 49ers would only owe him a $1 million roster bonus. So March 14 is essentially the deadline for Smith. He's not going to be with San Francisco any longer than that