There will be stops (1 or 2) but time to walk around is limited. Ive done such a Free Tour many years ago (on my very first trip via SIN), it was really more like a brief glance of the city.
But it has changened since then, so my experience may be a bit outdated.

Oh, you are flying in on SQ325. I usually take that flight for the product (i love SQ and the B777300ER has the latest Y/C/F product, same as on the A380) and schedule (Arrival in SIN in the late afternoon suits me most). If you are flying mid-week and in Y, there is a good chance that the seat next to you remains empty, sometimes you can even fetch an entire row for yourself which adds to the comfort. The B777300ER in Y is 3-3-3 configuration (one seat less than competitors), in C 1-2-1 with the widest C-seat on this planet.
I love SQs service (which some here on FT claim to be robotic) and its still above standard provided by European Carriers. The IFE is superb and offers a wide selection of movies, music and games (it was always top-notch in terms of selection but in the meantime there may be competitors which closed the gap).