I think they have to start off and complete the program ASAP. They have 1 A350 coming in next year and 4 the following year. Then Airbus will have ramped up their production of A350 and SIA should start getting a considerable amount of a/c on their order. They will need to start their cadet pilot program by 2016 to account for the 2 year training period and get pilots ready for when they start receiving a steady stream of the bulk of their A350 orders. Also if B787 10X program is on track then they are also scheduled to come in from 2018/19.
I would say they start off by Sept-Oct so that they complete the MPL(18 months) in time to end fiscal year 2015-16 and kickstart recruitment in FY 2016-17.