file: /proc/sys/net/ipv6/conf/interface/max_desync_factor
variable: net.ipv6.conf.interface.max_desync_factor
Official reference

Maximum value for DESYNC_FACTOR, which is a random value that ensures that clients don’t synchronize with each other and generate new addresses at exactly the same time. value is in seconds. Default: 600

Nb: per interface setting (where “interface” is the name of your network interface); “all” is a special interface: changes the settings for all interfaces.