The most I've been able to go recently (probably discoverable from my HN post history... cringe) is about 3 days. It felt great but I can't really keep it up. Also, elevated levels of booze was involved.
Does a smartphone count?
For anything other than taking phone calls or checking email, yes.
How checking email is not computer usage?
Yes.
I quit computers for two years only to resume a year ago. Very enlightening experience. I recommend it.
I, like many here, would find it difficult to pay the mortgage if I did that!