Well… I’m not sure what you mean by “completed”.
To reach Lvl60 in less than 1 year is realistically possible, using certain techniques (some people consider them as cheats, but that’s another discussion).
https://community.wanikani.com/t/vanilla-wk-in-354-days-and-a-short-guide
On the other side, if by “completed” you mean to burn every kanji and vocab out there… I’ll tell you my friend, that is nonsense 
It is a normal thing for everyone to forget some reading or meaning sometimes so, you would probably need between 6 to 12 additional months to complete WK.
https://community.wanikani.com/t/scaling-the-1569-review-mountain-epilogue-four-months-later
Finally, let me say that WK is certainly not the end for your Japanese study. This is just a tool to read Kanji. You will need to enroll in other activities to be able to communicate in an effective way (grammar study, reading, listening, pronounciation practice, etc.)
See you around 