Your script will most likely run out of time (default time limit is 30 seconds I believe...)
However, there are ways to get around this like ignore_user_abort() and set_timelimit(), etc etc...
Down to the point... It is generally not a good idea to keep a script running like that. As chris mentioned, you can use cron on a linux host or ms task scheduler on windows host (maybe if your host would set it up for you atleast...).
Besides, leaving a script always running (and from your web browser/ http server) will hog resources...