Visit visa is valid only for once entry, once u enter Qatar u can be here for a month as most of visit visas are valid for 1 month. once you r here u can extend it upto 3 months. after 3 months it gets lapsed so u need to leave the country.
for your next visit u can come back with a fresh visit visa. your present visa validity can not be extended beyond given date. in case u dont make it on time this visa is not valid.