U can come and go as long as ur Visa stamp in passport is valid. U will get new visa in airport that they call it i 94, and it says how long u can stay in usa, if it says 3 months then u must return to nepal in 3 months, but if u violate the i 94 duration of stay u can't come to usa even though u have valid visa stamp in paasport. Bottom line is as long as u obey i 94 rules u can travel up and down till 2019...