Extrinsic value is your time value and depends on what could happen to the price of the underlying over the time range the option contract covers. The two main variables will be time to expiration and IV.

As days to expiration goes to zero the extrinsic value drops and as volatility goes up or down, extrinsic value follows. IV is a measure of how market participants view the potential change in price and an option seller will raise the price due to the increased risk.

The option contract also changes value faster near expiration due to gamma. This is the primary reason that many option sellers will close contracts at least a week before expiration. Theta strategies are fairly easy to defend (depending on the delta one sells) if they turn against you. Rolling out and/or up/down is the standard way to defend a position. The wonderful thing about theta strategies is the clear rules that define. Simple if/else blocks can effectively manage a simple short option position.