A new diver will go through air a lot quicker than an experienced diver, so don’t worry about consumption so much. As you become more experienced and comfortable under water, your respiration rate will slow and you’ll breathe more deeply, which will increase your bottom time. My initial SAC was around 1.0 SCFM, now I’m down to less than 0.3 SCFM (standard cubic feet per minute) on easy dives. If you use a dive computer and download software, you can determine this consumption rate.
As for a tip to decrease consumption, try to think of your breathing cycle in thirds. First third - inhale. Try not to gulp, just sip the air. Second third - exchange. If you don’t allow the air you just inhaled to exchange carbon dioxide for oxygen in your lungs, you’re wasting it. So after breathing in, allow the air to sit in your lungs a few seconds. DON’T HOLD YOUR BREATH. Just don’t exhale immediately after breathing in. Final third - exhale. Try to S L O W L Y exhale, not just blow it all out at once. Let it dribble out. You’ll need to practice this technique but all experienced divers do it in some fashion. Watch an experienced divemaster’s respiration rate and try to match it - you’ll find there is a significant difference between the exhale time and inhale/exchange time.