A jam sandwich falling under gravity will accelerate at g=9.8m/s^2, ignoring air resistance. Acceleration is d^2x/dt^2 = v dv/dx = g. Integrating, v^2/2 = g(H-x), if the sandwich is dropped from rest at height H; its terminal velocity v = sqrt(2gH). Since the terminal velocity is the initial velocity plus acceleration times time (v = u + at), the time taken to fall is sqrt(2H/g). If the building is 20 storeys tall, and allowing 2.5m per storey, that makes H=50m. So, t = sqrt(2*50/9.8) ~ 3.2 seconds, hitting at about 30m/s, 110km/h or 70mph.
Says kensson, when asked about that Scottish song about throwing sandwiches from windows.