Page Life Expectancy doesn't tell you how much memory is being used, just a rough idea of how long a page in memory stays in memory.  To see h ow much memory is being used try queries like these:
-- system level info 2008
SELECT cpu_count/hyperthread_ratio AS physical_cpu_count,
    physical_memory_in_bytes / 1048576 AS physical_memory_meg,
    bpool_committed / 128 AS bpool_commited_meg,
    bpool_commit_target / 128 AS bpool_commit_target_meg
  FROM sys.dm_os_sys_info

-- SQL Server process memory
select physical_memory_in_use_kb / 1024 AS physical_memory_in_use_meg,
    locked_page_allocations_kb / 1024 as locked_in_memory_meg,
    page_fault_count AS sql_page_fault_count, memory_utilization_percentage, process_physical_memory_low, process_virtual_memory_low
  from sys.dm_os_process_memory

https://docs.microsoft.com/en-us/sql/relational-databases/system-dynamic-management-views/sys-dm-os-sys-info-transact-sql
https://docs.microsoft.com/en-us/sql/relational-databases/system-dynamic-management-views/sys-dm-os-process-memory-transact-sql