How often do you look at your phone? = 你多久看一次手机？ how often = 多久； How long does this process last? = 这个过程持续多长时间？how long = 多长（时间），sometimes I translate 'how long' to '多久' as well.

frequency: how often means 多长时间(做)一次（duo chang shi jian (zuo) yi ci), 做means do something.一次 means once. The structure in Chinese is: 多长时间＋verb+一次？ How often do you look at your phone? ＝多长时间＋look at 一次＋ phone =你多长时间看一次手机? for the duration: how long does something last= sth 持续多久 （chi xu duo jiu?）or 持续多长时间（chi xu duo chang shi jian） The structure in Chinese is: Something+ 持续多久/持续多长时间？ How long does this process last? = This process+持续多久／持续多长时间？ ＝这个过程持续多久？／这个过程持续多长时间？