It takes about 6 hours to harden sufficiently, and for the 1st minute or so I gently shook the tile so that air bubbles would come to the surface. I didn't get back to this for a week so these molds had a full week of curing time. Overkill, but since this was my first time doing this, I didn't want to rush things.
The legos came off easily, exposing the mold in all its glory. There was a little residue on the bottom, which was no problem, but I felt that the detail did not come out- in fact I thought they looked lousy ! Also, for the gun pits I used thin wood for the supports I had to dig these out of the mold, as they refused to stay embedded in the clay when I removed the mold from the tile. I had my mold for casting, and I thought I'd screwed up, but decided I needed to go on anyway and see how these babies cast up.
I used an acrylic casting compound (also sold by the same company) that would harden in 2-3 minutes (it is a 1:1 ratio so 1 ounce Part A and 1 ounce Part B). It starts out clear as you can see, then gets milky after a few minutes. At that point its close to being able to be removed from the mold.
I was surpised that the detail came out as well as it did. I popped them out when they were a little soft, which allowed me to easily "trim" some of the edge off them quite easily where the compound went over the edge of the mold. One of these I used wood for the posts, the other rectangles made from clay.
The detail came out extremely well (as far as my very bad attempts at texturing could be), and I was very pleased with the results. I can now cast up a boatload of gun pits for my 10mm Civil War combined Ironclad/Land operations game. After I get a bunch ready, I'll paint them up and post what the final result.