As noted, there is some variability in the warfarin dose to achieve the target INR for a given person. Each dose of warfarin tends to have a duration of action of 2 to 5 days and a half-life elimination of 20 to 60 hours. The INR measurement is the net combined effect of doses from the previous few days. This is why it can take a while for the INR to move up to the target level upon initiation of warfarin.
One other important consideration is the effect of other medications. I had some a-fib episodes during my post-op stay following my AVR and was prescribed amiodarone. The onset or duration of action of amiodarone varies considerably from a few days to several weeks. As the effects of the amiodarone builds up, it can have a substantial effect on how the warfarin is metabolized and typically results in a reduction of how much warfarin is needed to achieve the target INR.
As an example, I was initiated with a daily dose of 5 mg warfarin and a target INR of 3.0. My INR one week after my AVR was 1.5 and two days later it was 2.3. However, one week later, my INR had jumped up to 7.0. I was told to skip two days of warfarin followed by a 2.5 mg dose, and I indulged in eating turnip greens and was back down to 2.2 INR three days later. My amiodarone dose was reduced later that week and eliminated altogether a month later. My warfarin dose was gradually increased over the next few months as its interaction with amiodarone mitigated over time.
Since his afib is basically constant as far as we know, he will probably be on it until that is dealt with. We are still hoping he is among the 94% who naturally clear the afib as their heart heals. It's been less than 3 weeks since surgery, even if it feels like a lifetime.