Around every 6 months, tops. But sometimes more.
Like last year:
Started the year with the LG G4, upgraded to Sony Xperia Z5 (what a piece of crap), then sold it, used my old Galaxy S4 while I waited for the Galaxy S7edge, then upgrade to iPhone 7 Plus (sold it after 3 weeks, what a piece of schit apple is) and then got the LG V20
This year:
Started the year with the V20, upgraded to the S8+, and just put in my pre-order for the Note 8... might still get the Huawei Mate 10 when it comes out before year end.