The velocity will keep decreasing and it will reach 0 and then it will reverse.
So just before reversing, the velocity will be 0.
Initial velocity = 48 m/s
final velocity = 0
acceleration= -1.5 m/s²
v= u + at
⇒0 = 48 - 1.5t
⇒ t = (-48) / (-1.5) = 32s
At 32s, it will be 0. So velocity will be reversed just after 32s.