How fast should the flow rate be through a peripheral iv?

That depends on the age and weight of the patient and their condition.

Basic maintenance rates are:

100 ml/ kg/ hr for the first 10 kg

plus 50 ml/kg/ hr for the next 10 kg

plus 25 ml/kg/ hr for every kg above 20 kg.

If the person has a fever, shock, dehydration or other factors this may need to be increased, possibly to as high a 1000 ml/ hr or more. Other factors, such as Heart disease or renal failure may make it advisable to give it more slowly.