Boatman will stay at the jetty till 8 to 9plus usually.. if there is a crowd, they may ferry till later. But normally at low peak timing, you will need to wait till there a full ferryload before the ferry will move.. Best to arrange with the boatman if you are leaving the island late.
Alternatively, you can pay for the full ferryload and the boatman will bring you over too.
1pax trip is $12, Max is 12 people. So if you pay the full amount of $24, the ferry will bring you across if regardless of how many people there are as long as it do not exceed 12 pple.
Do consider staying over nite, Ubin is beautiful at nite too. I used to camp there about once a year. =)