It'll depend on your fitness level and the weather. On the morning of the summit, you'll need to get to the summit checkpoint by 5pm in order to be cleared by the local rangers for summit. If you do not arrive in time (due to fitness level or other reasons), you will not be allowed to proceed to summit. In addition, there are a handful of days each year where the summit climb is cancelled by Mt Kinabalu Park due to dangerous weather conditions. Cancellations happen if there is torrential rain and it's deemed dangerous for climbers to make the slippery summit climb. Cancellations are rare, and no refund will be issued by Mt Kinabalu Park for any cancellations.