How long have you been using it ? Hong have you used it ? What's the difference betwen these two sentences ?

They can mean the same thing, if you're talking about a long-term action (days, weeks, months, etc.). For example, "(For) How long have you been using your phone as a calculator?" = "(For) How long have you used your phone as a calculator?"
But if you're talking about a short-term action (minutes, hours), they mean different things:
"(For) How long have you been using the iPad today?" means that you're still using it right now.
"(For) How long have you used the iPad today?" means that you stopped using it recently today.

View more