Background: Over the past five decades, warfarin, an anticoagulant drug, has been used in patients with Atrial Fibrillation (AF) for the prevention of embolic stroke. Many guidelines suggest that AF patients who have had warfarin treatment initiated, should also be supervised to provide optimised quality control of the warfarin by monitoring the Time in Therapeutic Range (TiTR). If patients achieve a high TiTR, warfarin treatment is appropriate and should not be switched to Novel Oral Anticoagulant drugs (NOACs). Moreover, the guideline from the Thrombosis and Haemostasis society of Australia and New Zealand (THANZ) suggests that AF patients who have had TiTR > 65% for over a three months period are stable on warfarin treatment and should not be switched from warfarin to NOACs drug treatment. However, international guidelines do not indicate what the suitable duration for observing the quality of warfarin control should be and there is no study about the duration of warfarin trials in patients with AF. Aim: This research aimed to address the Time to Stable Therapeutic Range (TtSTR) and determined the factors that affected this. In addition, this research also aimed to assist in the screening for patients’ suitability for warfarin treatment and patients who might not achieve good quality control from warfarin. Furthermore, this research aimed to provides a suitable duration recommendation for a warfarin trial, which has not previously been provided. This was to ensure that the actual TiTR observed at the time of decision making is a true reflection of the potential TiTR that a patient is likely to achieve. Method: Retrospective data were collected between November 2007 and October 2014 from Sullivan Nicolaides Pathology, Queensland. International Normalised Ratio (INR) measurements were collected and used to calculate the individual TiTR at different specific time points: 1, 2, 3, 4, 5, 6, 9, 12, 18 and 24 months by using Rosendaal's linear interpolation algorithm. The TiTR results at different specific time points were assessed in order to discover the Time to Stable Therapeutic Range (TtSTR). Patients were divided into two groups according to low or high (≤ median of TtSTR or ˃ median of TtSTR, respectively) TtSTR. The independent t test was used to compare the mean TiTR between these two groups at each time point. Moreover, this study investigated the factors which were associated with higher TtSTR by using a multivariate logistic regression analysis. Results: The median of the TtSTR of the eligible 566 AF patients was six months, and almost 57% of patients had TtSTR ≤ 6 months. Only two factors, namely IRSAD (Index of Socioeconomic Advantage and Disadvantage) rank 9 and carbamazepine, had a significant relationship with TtSTR > 6 months. Patients who were concurrently treated with carbamazepine during the first six months of warfarin treatment had 15 times the risk of TtSTR > 6 months when compared to patients who were not treated with carbamazepine. On the other hand, patients with IRSAD rank 9 had a decreased risk of TtSTR > 6 months (OR = 0.392, p = 0.022) when compared with patients with IRSAD rank 1. Conclusion: The results of this study indicate that the patients who achieve TtSTR within 6 months could maintain good warfarin control (TiTR ≥ 65%) in the next 18 months. Moreover, it could be suggested that a six month period is the minimum duration of a warfarin trial and a 12 month period might be the maximum duration for Australian patients with AF. However, the factors that associated with TtSTR cannot be indicated due to the limitation of this study.