How frequent to mythical ancients drop?

I just recently started playing about 2 weeks ago, rushed through the campaign and the vessel of hatred campaign as well. I’m just about paragon 190ish and am about ready to do a 35 rift and haven’t had one mythical drop yet. Almost 40 hours of game play. Does it take a while, also I’m currently playing on T1. Have done hoards, whispers, rifts.