I just learned about this feature.
Example using SQL*Plus 11.2
12:56:58 MDINH@db01:PRIMARY> timing start test1
12:57:07 MDINH@db01:PRIMARY>
12:57:21 MDINH@db01:PRIMARY> timing start test2
12:57:23 MDINH@db01:PRIMARY>
12:57:36 MDINH@db01:PRIMARY> timing stop test2
timing for: test2
Elapsed: 00:00:16.33
12:57:40 MDINH@db01:PRIMARY> timing stop test1
timing for: test1
Elapsed: 00:00:48.96
12:57:56 MDINH@db01:PRIMARY>
