I'm told that IRCC will only issue test invites at times that are
prime numbers when you take the four digits that make up the time in 24hr format. So here is a list of the possible times that you might receive a test invite:
00:02, 00:03, 00:05, 00:07, 00:11, 00:13, 00:17, 00:19, 00:23, 00:29, 00:31, 00:37, 00:41, 00:43, 00:47, 00:53, 00:59, 01:01, 01:03, 01:07, 01:09, 01:13, 01:27, 01:31, 01:37, 01:39, 01:49, 01:51, 01:57, 02:11, 02:23, 02:27, 02:29, 02:33, 02:39, 02:41, 02:51, 02:57, 03:07, 03:11, 03:13, 03:17, 03:31, 03:37, 03:47, 03:49, 03:53, 03:59, 04:01, 04:09, 04:19, 04:21, 04:31, 04:33, 04:39, 04:43, 04:49, 04:57, 05:03, 05:09, 05:21, 05:23, 05:41, 05:47, 05:57, 06:01, 06:07, 06:13, 06:17, 06:19, 06:31, 06:41, 06:43, 06:47, 06:53, 06:59, 07:01, 07:09, 07:19, 07:27, 07:33, 07:39, 07:43, 07:51, 07:57, 08:09, 08:11, 08:21, 08:23, 08:27, 08:29, 08:39, 08:53, 08:57, 08:59, 09:07, 09:11, 09:19, 09:29, 09:37, 09:41, 09:47, 09:53, 10:09, 10:13, 10:19, 10:21, 10:31, 10:33, 10:39, 10:49, 10:51, 11:03, 11:09, 11:17, 11:23, 11:29, 11:51, 11:53, 12:01, 12:13, 12:17, 12:23, 12:29, 12:31, 12:37, 12:49, 12:59, 13:01, 13:03, 13:07, 13:19, 13:21, 13:27, 14:09, 14:23, 14:27, 14:29, 14:33, 14:39, 14:47, 14:51, 14:53, 14:59, 15:11, 15:23, 15:31, 15:43, 15:49, 15:53, 15:59, 16:01, 16:07, 16:09, 16:13, 16:19, 16:21, 16:27, 16:37, 16:57, 17:09, 17:21, 17:23, 17:33, 17:41, 17:47, 17:53, 17:59, 18:01, 18:11, 18:23, 18:31, 18:47, 19:01, 19:07, 19:13, 19:31, 19:33, 19:49, 19:51, 20:03, 20:11, 20:17, 20:27, 20:29, 20:39, 20:53, 21:11, 21:13, 21:29, 21:31, 21:37, 21:41, 21:43, 21:53, 22:03, 22:07, 22:13, 22:21, 22:37, 22:39, 22:43, 22:51, 23:09, 23:11, 23:33, 23:39, 23:41, 23:47, 23:51, 23:57
For the sake of your sanity, make sure to only check your email at these exact times, otherwise you're wasting your effort.