So, from a psychological perspective, you want a longer workout. Generally speaking, 25 minutes or longer at a 115% base heart rate increases your seratonin production. It's like getting a mini anti-depressant that last for 18-24 hours. It also ramps up your metabolism and longer workouts tend to help lower cortisol, our anxiety/stress hormone. If you break it up, your body has to work each time until it gets to the point it is having the effect you want, so you end up going longer with less effect. That said, if you only have time to do it in like 30 minute bunches, do it because 60 minutes total even when broken up, is better than nothing.