The Skyhawks were shipped to France in 1998 and the Singapore Air Force will stay in Cazaux untill 2018.
According to AFM, 10 TA-4SU arrived in 1998 and 8 A-4AU in 1999. But it says that 2 single-seaters have already been mothballed. So the total number of Skyhawks is 16, wich will never return to Singapore. When they reach the end of their days they will probably be scrapped in France.
But I don't know how long they will be kept in service, since the original airframes were delivered in the 70s.
Little 2007 update, another AFM (#232 July 2007) feature on this topic. Currently the number is still 16, 10 TA-4SU and six A-4SU. All maintenance is carried out at Cazaux, thanks to co-located Singapore Technologies Aerospace branche (30 engineers on-site to service the type every 300 hours). It says there are spare airframes in Singapore (so they weren't scrapped after retirement) but Sqn CO says they're not needed to keep the current fleet in service. The Skyhawk is said to be able to support the training need for several more years to come. ST Aero was awarded a new support contract in March 2007, for full support (maintenance, flightline ops, engineering, logistics) of the detachment for a period of four years. So they'll stay in service for at least four years from then. Replacement plans are still at a "very preliminary stage".
94 pilots graduated, 19,200 hours flowns since its inception in June 1998 to December 2006.
By the way, those two high houred 'mothballed' examples you mentioned have made their way to two museums. (it says donated in 1996, but I guess that's incorrect, or just donated on paper)
Anybody knows when RSAF plans to replace F-5S/T?
I presume when the F-15SG starts to get on strength with the RSAF. So I estimate in 2009 or 2010... depending on speed of deliveries and conversion training. Anybody else with details?