SIN to HK only 3 hours plus...less than 4 hours.
can allow you to watch 1 and half movie if you taking SIA.
if take budget, just enough time for you to chat up the girl beside you and arrange for date that evening for bonking...hee
One thing though, please be prepared to queue long long to clear immigration...and put up with bunch of PRCs who hogged the lines...they have dedicated lines for PRCs but they like to cram up the lines for normal visitors...KNN
If you are buying ciggy for frens in HK, buy before going to queue for immigration, no duty free after that point