By RAW the average length of a combat encounter between equal level PCs and monsters will rise from 6 rounds at level 1 to 16 rounds at level 10 to 23 rounds at level 20.
By RAW the average length of a combat encounter between equal level PCs and monsters will rise from 6 rounds at level 1 to 16 rounds at level 10 to 23 rounds at level 20.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.