Everything is taxable. For Singapore tax rates, you can check Singapore's tax revenue's website:
www.iras.gov.sg.
SIA Cargo is part of the IRAS' auto inclusion scheme, meaning the company will submit all our taxable income to IRAS. Then, you can submit what relief you are entitled to. When the tax men will then calculate your IOU by June or July, you will have to pay up either as a one-time lump sum, or by monthly deduction via your bank account.
One way to reduce your taxes, legally, is to apply for the Non-Ordinary Resident (NOR) status, which you can for the first five years, as explained by millerscourt.
As what burnden mentioned, our rosters are very unstable, or as what management likes to say, very dynamic. So you could spend 10 days in Singapore in a month, or maybe 20 days.