This depends on the index scheme. If indexed 2 ways, assuming that the graph will always be stated in queries, this is 31 bytes.

With 4 indices, supporting queries where the graph can be left unspecified (i.e., triples from any graph will be considered in query evaluation), this is 39 bytes. The numbers are measured with the LUBM validation data set of 121K triples, with no full-text index on literals.

With 4 indices and a full text index on all literals, the Billion Triples Challenge data set, 1115M triples, is about 120 GB of database pages. The database file size is larger due to space in reserve and other factors. 120 GB is the number to use when assessing RAM-to-disk ratio, i.e., how much RAM the system ought to have in order to provide good response. This data set is a heterogeneous collection including social network data, conversations harvested from the Web, DBpedia, Freebase, etc., with relatively numerous and long text literals.

The numbers do not involve any database page stream compression such as gzip. Using such compression does not save in terms of RAM because cached pages must be kept uncompressed but will cut the disk usage to about half.