Source: (WOT Chinese official site)
Due to the law in China, loot boxes are required to show their drop rate. It is unknown if the same drop rates apply for other servers.
Here is the drop rate for items in the loot boxes:
Epic Items: (Bisonte C45, GSOR 1008, ISU-152K, Bourrasque or 3D Styles): 8.4%
High-Value Items (Other lower tier premium tanks): 11.66%
Other In-game Item (Premium days, gold, silver): 85.04%.
For every 50 large boxes, you are guaranteed to get one EPIC Items. E.g. If you opened 49 large boxes and got none of the EPIC items, the 50th box you open should contain one of the EPIC Items.
Again, it is only for Chinese Server, there is no guarantee that the similar drop rate applies for all servers.