In a word: Very!
Many K-1 visa applicants often ask if it is okay if they have not seen each other in two years. Many think it is okay – especially when a couple has known each other for many years, have seen each other, and have even lived together for a time. Unfortunately, that is not the case.
The USCIS is very strict at requiring a K-1 visa couple to meet during the two years immediately prior to filing the K-1 visa. It is not sufficient to meet after the petition is submitted, or to have meet two years and one day prior to filing the K-1 fiance visa. A couple must meet within the two years immediately before the application is filed.
The USCIS does provide for two types of waiver to the two year meeting rule: the extreme hardship waiver and the cultural prohibition waiver. However, both are difficult to obtain, and a consultation with a fiance visa lawyer should be scheduled.