Albert Barnes Commentary 1 Kings 9:10

"And it came to pass at the end of twenty years, wherein Solomon had built the two houses, the house of Jehovah and the king`s house" — 1 Kings 9:10 (ASV)

The “twenty years” are to be counted from the fourth year of Solomon, the year when he commenced the building of the temple. They are made up of the seven years employed in the work of the temple (1 Kings 6:38) and the thirteen years during which Solomon was building his own house (1 Kings 7:1).