The bricks take about default time to come back (30-40 seconds)
Its not specifically 300 bricks but around "alot", let me actually show you my "Demo dirt pile"
This is my physics test build, it has a little over 100k amount of bricks, exact number is there on the pic. Comes with controls to launch a rocket straight up and back down.

This is the aftermath. My Laptop (At the time of taking this picture) can actually handle the massacure of the bricks. Everything flows beautifully and leaves a hole in the middle. The problem is that instead of having the bricks stay ontop of the bricks/map, it falls through...

Here is how it looks on the inside
