Astronomical objects are structured hierarchically and so not everything is direct instance of Q6999 (unlike scholarly articles).

Considering all subclasses of Q6999, the number of astronomical objects form ~9% of all Wikidata entities. (sparql query)
And an approximation of the number of triples 'related to' these entities is 7.5% (~1B) of all Wikidata triples. Approximated from top 10 subclasses (which are 7% of all entities)

Counts of queries and triples for astronomical objects were done here: Wikidata_Subgraph_Query_Analysis, along with the top ~300 large subgraphs.
For the specific case of Astronomical objects (and only astronomical objects), a list of all its subclasses was obtained and manually inspected for relevance to astronomical objects. The subclass list also consists of subclasses of subclasses and so on.

• Percent of triples: 8.7%
• Percent of entities: 8.9%
• Days to recover: 245
• Query count: 2.5M
• Percent of queries: 1.3%
• Percent time of all queries: 0.5%