Removing the buzzer is not advisable-you should ask a knowledgeable technician to inspect your ups and carry out the necessary repairs to fix this issue.Repairing it won't be too difficult i think-it'll be better still if you can contact proview's service centre for getting it repaired.
Keeping the UPS running for atleast 3-4 hours will ensure that the batteries have recharged fully-depending on your PC's power consumption,the backup time may well range anywhere between 5 mins-15 mins or more.