edit: also how long could this go on (1-3 times) with minimal damage? if you do it for years and years at such a frequency will you inevitably become ill? or could you live to old age with this behavior...? this is not for myself, so no worries. just out of curiousity and worry for others.
I'm looking for more of an average (or a survey of individual experiences, even studies) than anything exact because, of course, it varies from person to person.