Aha! So written it into script and ran in terminal:

for i in `seq 1 $SECONDS`; do
	***Line here outputs the string to text file. It's long, so removing it from forum for now ***
	sleep 1
	if ! ((i % 5)) || ((i == 1)); then
		echo "$i, would execute clean script here"
		sudo sh -c 'echo 3 > /proc/sys/vm/drop_caches'

The purpose of the echo is for debugging whilst writing, and the i==1 so that on first purge that a sudo password is required. Therefore my thinking is this: run the purge on first time output, enter the SU password which will then be remembered and redo the purge every Nth second. Set as the MODULO , obviously not to be purged eveery 5 seconds - I figure every 10min to do this. Alternatively if the cache gets above 50%, I could do it then...