The short answer is: not often at all. Smartphones are designed
to remain on all the time, broadly speaking. After all, the idea of a 'mobile
phone' is that you can always be contacted. (Smartphones are phones, remember?)
Of course you will need to restart your phone from time to time.
You will need to restart to install OS updates, if you are lucky enough to get
them. And on occasion if things are buggy and slow a restart will help.
