create 2 variables>> counter and temp counter with values 0
You can use an Untill activity
within the untill activity :
use getmeta data activity to get the number of files
then use an if activity>> if number of files is 10 then nothing else use wait activity for 15 mins and then use set variable activity to add 1 to temp counter and then use set variable activity to assign counter variable the temp counter variable.
untill activity condition would be either the number of files is 10 or counter variable value is 8 ( reason for 8 is 2 hr window with 15 mins intervals)
after existing if the count is not 10 then use fail activity to throw error