How often can you LTA claim it?

Twice in a block of four calendar years.
In case you were unaware, LTA is not related to when you started your employment. The government fixes blocks of years.
These blocks are not financial years (April 1 to March 31); they are calendar years (January 1 to December 31).
The current block is 2006-09: January 2006 to December 2009.
The earlier one was from 2002-05: January 2002 to December 2005.
During this time period, a person is entitled to two LTA claims.
A person can get an income tax exemption for two journeys in a block of four calendar years. But he can make a trip only once in year.