Choosing an appropriate voltage regulator to limit power consumption
You need to work out a power management strategy: Will each circuit be powered all the time? Can the MCU be put in power-save modes and if so, how often? Should you use extra components to power off power-hungry parts completely? For example, you might be able to only power the RTC and wire it up to power up the voltage regulator at the right times, for a very low-power design. For example, Kevin Darrah on Youtube explains his work with extreme power-saving.

In parallel, you need to make a spreadsheet of the power consumption of each component. This depends on the choice of regulator etc. Look up the current consumption of each component in the datasheet and consider how it contributes to the whole power budget. For example, you will find that a reasonable RTC draws very little current (a few microamps or less). You can use a boost regulator to power from a one-cell battery, or a linear or switching regulator to lower a higher-voltage battery. Each regulator comes with a cost and its own efficiency ratio at the level of current draw that you have.

The battery time in hours will simply be capacity in mAh divided by total current draw in mA.

As you start to see the different design options you have, you will gain some insight into the relative work needed for each design and whether the work is worth your time. It's not possible to outline the options here, as they depend on many factors such as how often you need measurements, acceptable work and cost etc.