How can I find time of an insertion ?

 You can find INSERT time using fn_dblog() function looking at [Begin Time] column of the corresponding begin transaction (with INSERT name) in the result set.

The tricky thing here is that the column AllocUnitName of this row (LOP_BEGIN_XACT) is NULL, while [Begin Time] is NULL for rows corrensponding to LOP_INSERT_ROWS where AllocUnitNamecan be found.

So you should use the fact that the transaction is the same, and once you individuated the rows of interest using your query that filters for 'LOP_INSERT_ROWS', you should use [Transaction ID] to find the time:

select  [Begin Time]
from sys.fn_dblog(NULL,NULL)
where [Transaction ID] = '0000:cb5f3b8a'

 -- put here the [Transaction ID] found using your query
and Operation = 'LOP_BEGIN_XACT';