How often do siege creeps spawn?

Siege creeps spawn every 7th wave, starting at 3:00 (the 7th wave). At 45:00 into the game, each wave with a siege creep will instead spawn 2 siege creeps.

Because the first wave spawns at 0:00, the first siege creep is at 3:00, then every 3:30 afterwards.

Additional Information

Each wave will spawn an additional melee creep at 15:00, 30:00 and 45:00. At 45:00, each wave will spawn an additional ranged creep.

Thus after 45:00, each wave will spawn 6 melee creeps, 2 ranged creeps and 0 or 2 siege creeps (siege creeps still only spawn on every 7th wave).

Melee and ranged creeps get stronger (damage and health) every 7:30, maxing out at 3:45:00. When you destroy all of an enemy's barracks, the creeps instantly reach maximum strength, and siege creeps get a damage boost.

Source