It feels random.
Most of the time I can login 20 characters at once and then the other 5 after few minutes, this doesn't always work.
I did daily quests on 25 characters in around 40 minutes.
Then I tried logging in 25 different characters and only 6/25 characters got in.
After 3-5 minutes it allowed 3-5 characters and then again.
After 20+ minutes I still didn't have all my characters logged in and missed WG.
Any official word about this would be nice.