Depends on clock speed. The 'standard' way of delys longer than the time will cover at said frequency, is to count timer incidents in the corresponding timer interrupt.

counter ds 1

init:
set counter to 150
set Th, Tl for 20 msec
start timer

main:
if counter = 0 time has passed



timer ISR
stop timer
if counter = 0, exit
decrement counter
set Th, Tl for 20 msec
start timer
exit

Have fun,

Erik