With Enterprise Vault 10 came a whole new 64 bit indexing engine. Along with that is another slight change and that is in the indexing levels, there used to be three levels :
Now there are just two brief and full. In most documentation Symantec suggests that brief indexing requires about 4% of the overall size of all your archived data, and full needs about 13% (See this link).
So what does this mean in real life?
If you have 4 Tb of archived data (or it is predicted that is what you'll have in say 6 months) then you'll need the following storage for your indexes:
4 Tb = 4000 Gb (thereabouts)
4000 * 0.13 = 520 Gb
520 Gb of storage for your indexes.
Airing on the side of caution, if you allocated 15% of storage instead then you'd see the need for:
4000 * 0.15 = 600 Gb