How much vacation time is typical during a PhD in Singapore?
I got my Ph.D. at NTU.  Ph.D. students would generally be getting 21 days of leave for each calendar year (i.e. Jan-Dec) in both NUS and NTU (see links below).

With a 5 day work-week, that means a maximum of 4 weeks + 1 day of leave.  This can be increased if the period coincides with one or more of Singapore's 11 public holidays (as published by the Ministry of Manpower).  No-pay leave may also be possible, if the Ph.D. supervisor agrees.

Note that Ph.D. students would be still be working during the undergraduate summer break vacation periods.

References:


http://nus.edu.sg/ngs/leave_ngss.html
http://www3.ntu.edu.sg/asis/gso/rsls_leave_policy.pdf
http://www.cee.ntu.edu.sg/Students/Graduate/PhD_MEng/Documents/Leave%20Scheme.pdf