Because Raspberry Pis are built to work for lengthy periods of time, you can leave them on all the time. It is, however, not advised to do so. The item will overheat and be destroyed if kept on for an extended period of time without sufficient cooling. Its lifespan is also shortened if it is left on all the time. If you want to keep your Raspberry Pi on all the while and not risk damaging it, you'll need to use a linear power source. DC adapters or any decent quality battery supply must be used to regulate this.