Four hours plus depending on the bus operator, occasionally much slower if you travel at a particularly bad time when the roads and immigration are jammed (Chinese New Year, Hari Raya, etc).

There are huge differences between the operators/buses and spending an extra $10 for a better one is usually well warranted; make sure they use the Second Link (much faster than going via JB) and have a toilet on board (less stops). The nicer buses have lots of single seat options. The companies are locked in an endless arms race, but the major players are listed here:

http://en.wikivoyage.org/wiki/Singapore#By%20bus

I've found Aeroline and Transtar to be good luxury options, and First Coach to offer a speedy but fairly no-frills ride. YMMV.