Thank for the help Somebody! I ended up going with a totally different method:
player_get_kill_count since it not only requires someone to be in map a while, it also requires them to have a set kill count.
Works like a champ and better fits the theme of the scene prop.
If anyone is looking to do something similar to this thread title these were discovered with Somebody's help. These may help someone else find their path:
1. (player_get_slot, ":player_join_time", ":player_no", slot_player_join_time), - could be used to do something based on join time
2. scene_prop_enable_after_time - when used with a scene prop in module_scripts file it allows a delay before activation
3. store_mission_timer_a/b/c along with scene prop use trigger - if looking for time passed since mission start
Feel free to post any other methods for time management in this thread so others can benefit in the future!