InnoDB Buffer Pool Hit Rate As given at http://cherry.world.edoors.com/COBFKUqnUdBY one can obtain buffer hit ratio by following sql query:

SELECT round ((P2.variable_value / P1.variable_value),4), 
P2.variable_value, P1.variable_value
FROM information_schema.GLOBAL_STATUS P1,
information_schema.GLOBAL_STATUS P2
WHERE P1. variable_name = 'innodb_buffer_pool_read_requests'
AND P2. variable_name = 'innodb_buffer_pool_reads'; 


QUESTIONS


From what period are buffer hit ratio given by that query? From start of database engine till now?  
Is there possibility to obtain buffer hit ratio from given period of time? (for example: last 10 minutes)
That's the Hit Rate since Uptime (Last MySQL Startup)

There are two things you can do to get the Last 10 Minutes

METHOD #1

Flush all Status Values, Sleep 10 min, Run Query

FLUSH STATUS;
SELECT SLEEP(600) INTO @x;
SELECT round ((P2.variable_value / P1.variable_value),4), 
P2.variable_value, P1.variable_value
FROM information_schema.GLOBAL_STATUS P1,
information_schema.GLOBAL_STATUS P2
WHERE P1. variable_name = 'innodb_buffer_pool_read_requests'
AND P2. variable_name = 'innodb_buffer_pool_reads'; 


METHOD #2

Capture innodb_buffer_pool_read_requests, innodb_buffer_pool_reads, Sleep 10 minutes, Run Query with Differences in innodb_buffer_pool_read_requests and innodb_buffer_pool_reads

SELECT
    P1.variable_value,P2.variable_value
INTO
    @rqs,@rds
FROM information_schema.GLOBAL_STATUS P1,
information_schema.GLOBAL_STATUS P2
WHERE P1.variable_name = 'innodb_buffer_pool_read_requests'
AND P2.variable_name = 'innodb_buffer_pool_reads'; 
SELECT SLEEP(600) INTO @x;
SELECT round (((P2.variable_value - @rds) / (P1.variable_value - @rqs)),4), 
P2.variable_value, P1.variable_value
FROM information_schema.GLOBAL_STATUS P1,
information_schema.GLOBAL_STATUS P2
WHERE P1.variable_name = 'innodb_buffer_pool_read_requests'
AND P2.variable_name = 'innodb_buffer_pool_reads'; 


Give it a Try !!!