In Britain we must let them know by 25 week's of our intention to take mat leave and the date we will be leaving work. If we don't they legally don't have to pay us or keep our job open. I have to let my employer know of my mat plans in 2 more weeka
Try to give them at least 2 weeks if you're requesting your ROE. There is no set time in advance really.
try to tell them as early as possible..so they'd understand what you're going thru..