Everyone reacts differently. Keep the same amount as long as the weight keeps coming off and the effects feel strong enough. Half a milligram is a low amount, so if it is already working there is no need to raise it yet. Only move up half a milligram when the current level stops delivering results.
I took my first 0.5 mg shot on the 12th and the next one a week later. I have dropped six pounds so far with no changes to what I eat or how I move. How do I decide it is time to go higher?