How far i can scrape my tongue?

Epiglottis: Your tongue ends at the epiglottis, so technically you can scrape to that point, but most people would be choking and gagging by sticking a tongue scraper that far down their throats. The circumvallate papillae is a good place to end, which is basically where the taste buds end and where the tongue starts curving down into your throat. Further may not really be beneficial or advisable.

Dictated by comfort: Your can use your tongue scraper as far back as you are comfortable and don't cause yourself to gag. Mucous can collect from post nasal drip and cause halitosis or bad breath. Removing that mucous will decrease morning breath.