I might get ridiculed for posting something so simple but I only just found out about this, so hopefully it might help at least someone else.
I was finding I was logging in with 5 minutes of an invasion to go and getting pissed off. Looked all over Wowhead for details of when these might occur to no avail.
Then looked at the homepage and it's right there.
World of Warcraft: Legion News
Alongside where it says World Boss gives the times of the next 3 invasions.
I might be the last person in the world to know about this - but putting it here just in case I'm not.