Set this option as an interval based in seconds. (1 - 9, being the acceptable parameters. Decimals to the 100th will work as well, i.e. [ 1.25 ] being the set number would generally amount to, in game, 1 second and 1/4th of a second.)
/edit, A quick note... the count-down to when the actually teleport will take place.. will only be activated while standing
IN the area in which the trigger_teleport is placed over. Leaving this area before the 9 seconds (or whatever set time) is up will cause the countdown to start over for that specific player.
Unless, of course, the teleport is activated through some other means, such as a trigger_multiple, multimanager, func_button, ect. Then, in which case, the count-down timer will start immediately after triggered and follow through until closed/canceled.