You can expect to run out of irish moss sooner :P
Seriously no worries. Look into whirfloc tablets whenever you run out of irish moss. They're more or less the same thing but in tablet form. 1 tab per 5-6 gal batch and you're g2g. Not that there is anything wrong with irish moss. Either way there is no need to be excessively worried about the amt, just grab a pinch and throw it in.