I searched for 15 minutes and to me the syntax still isn't clear... would this:
*/5 8,9,10,11,12,13,14,15,16,17,18,19,20 * * *
run the job every 5 minutes from 8(8am) to 20 (8pm)?
search no more, just type in shell man 5 crontab
and read...
hint:
*/5 8-19 * * * /path/script
per @madhatter comment, you should add another job
0 20 * * * /path/script
.