Let's say there is a timer for 15 second spawns. As soon as the map begins, the timer is cycling every 15 seconds for spawns. Back in competition we had a designated timer person on coms, that at the end of every 15 seconds would say "spawn time" to announce the spawn waves until the match was over. The spawn timer applies to the entire server. This was a tremendous strategy for knowing when the other team would spawn as well as our own team players.
Our clock watcher would also monitor enemy tank and plane spawns as well.
I know Mexican Eskimo and Rubulator touched on this briefly.
In a pub with tk punish and forgive on there is a variance will flux as well.