Given the size of Teldrassil in lore it would probably take a day or two at the very least. Even discounting how there were only something like 2 portals for one person at a time to go through at a time they were both in the middle of Darnassus, the time needed to even get the word out to the more outlying areas for them to start moving towards Darnassus would take hours at best, not to even get into how long it would take for them to walk there.