To naturalize, it'll likely take close to 5 years from now even with K-1 route. Let's produce an optimistic estimate.
* K-1 approval - 6 months
* Visa Interview, visa issuance - 3 months.
* AOS - 6 months
* 3 years as LPR before filing for N-400. Assuming she still has pending I-751 at that point when becoming eligible to file for natualization.
* N-400 processing - 8 months.
Total: 6 months + 3 months + 6 months + 36 months + 8 months = 59 months, which is give or take 5 years from start to finish if you file today.
But again, this is very optimistic. I've seen AOS taking easily 12-18 months and N-400 taking 12+ months. So overall it could be close to 5.5 years