HDB already got strict rules for subletting, the number is just to calibrate to ensure FW not using Condo as Dormitory.
Subletting period
The minimum subletting period for each subtenant must be 6 months per application. You are not allowed to sublet your flat or bedroom on a short-term basis as it may disrupt the living environment and pose security concerns for our residents.
The subletting period is indicated in the approval letter, up to a maximum of 3 years if all the subtenants are Singaporeans or Malaysians. The maximum is capped at 1.5 years for non-Malaysian non-citizen subtenants. Non-citizens refer to Singapore Permanent Residents and foreigners. You are required to reapply for approval each time you sublet your flat or renew the subletting application.
http://www.hdb.gov.sg/cs/infoweb/res...ng-regulations