From taransworld, we can have some stats. If you take the week of 10/02/2017 for a one-Mythic-exclusive week:
- with 100 VIP chests, you have 63% to obtain the Mythic
- need 300 VIP chests for a 95% to drop it
- almost the same for gem keys, 1000 for 63%, 3000 for 95%