Some people clean their phones daily or twice per day depending on how often you use or touch your phone.
During cold and flu season it's best to wipe your phone down with Clorox cleaning wipes twice or even 3 times per day to keep the germs off the phone.
I usually just wipe my phone and screen down on my android phone once daily or sometimes every few days depending on how often I touch the phones screen.
Mostly my finger prints get on the phone which makes it hard to see what's on the screen so I at least try to wipe the phone screen down everyday if I remember to do so.
You can buy a phone cleaning device online that you can put your phone in and it safely cleans the phone of germs etc but just wiping the phone down with a damp cloth and soap or even just some baby wipes or Clorox wipes will be enough to keep the phone clean.