file: /proc/sys/vm/dirty_expire_centisecs
variable: vm.dirty_expire_centisecs
Official reference

This tunable is used to define when dirty data is old enough to be eligible for writeout by the kernel flusher threads. It is expressed in 100’ths of a second. Data which has been dirty in-memory for longer than this interval will be written out next time a flusher thread wakes up.