With maintenance, basically forever. The Thames Tunnel, built in
1869, is still able to run subway trains through it, though they
had to close it 3 years for repairs.
I agree it might be a bit far stretched. But considering the fact
that Singapore is being overpopulated, the above ground structures,
skyscapers, roads, water areas (ponds, drains, etc) is getting
heavier and more stressful to the underground structures. And not
to mention other underground structures together like underground
sewages, cables, pipes, etc that will eventually make the
underground foundation more unstable.
And do not compare UK or other European countries with Singapore
since these countries are far less densely populated as per area.
It is just like comparing durians with apples.
Even if it doesn't collaspe, think of the long term problems,
leaks, cracks, etc. Long term maintenance and operations is still a
problem forever.