There are many factors besides contemplation of future punishment that influences whether people become more or less likely to commit a crime, so I would focus

There are many factors besides contemplation of future punishment that influences whether people become more or less likely to commit a crime, so I would focus on tackling the whole host of them, rather than moving that one lever. Mental health, poverty, education, urban planning, etc. all influence crime rates. For example, one factor contributing to the U.S.'s high drunk-driving rates is that the state of late-night public transit, combined with urban design, means that people routinely drive to bars, which sets up an obviously problematic situation. Plus, the empirical data on sentence lengths as a deterrent is fairly weak. People do seem to respond to likelihood of being caught: a 6-month sentence you are virtually sure to get is more of a deterrent than a 6-month sentence that you have maybe a 5% chance of being caught and sentenced to. But especially at the high end, there is little to no evidence that having prison sentences of 30 years is more of a disincentive than prison sentences of 10 years.