Decide how long you want a wave to last and set an event that begins when the game starts that waits x time then sets a variable to 1, repeats and sets to 0, repeat... and have your boss triggered however long after this variable is detected to be 1.
My side scrolling space shooter (in my sig) uses waves simply for time, I have 100 waves till final boss. I can adjust my wave length (pun) to decide how long it takes the player to reach the end to keep it from getting boring, or too easy. In mine currently every 7 seconds a new wave begins and on waves 25, 50, and 75 a boss spawns and on 100 the final boss spawns. I toyed with making it infinity style but realized pretty quick that creating new content and scaling up current enemies forever you eventually reach a critical mass with a shooter unlike an obstacle avoiding game where its the players concentration or rhythm that carries them through.