Complex sentence: Komarr's economy is dominated by the oligarch families, whose large fleets are sent on trade runs (lasting 5--12 months) to many areas throughout the galaxy, often returning with very large profits.
Split into simpler sentences: Komarr's economy is dominated by the oligarch families, who own large trade fleets. These trade fleets are sent on 5-12 month long trade runs to many areas throughout the galaxy, often returning with very large profits.