Consume 12v to 5v converter
A switching converter is not the best one for every application, for yours it probably is though.

To calculate the battery lifetime, you need to calculate the current which is drawn from the battery.

A switchmode converter will transfer the power with a certain efficiency, which is sadly not given for that device, so I'll assume some bad numbers.

$$P_{in} = \frac{P_{out}}{\eta} = \frac{5V*300mA}{0.7} \approx 2.2W $$

Even a bad switchmode power supply should be able to get 70% efficiency. So the power supplied by the battery is 2.2W, but you have the capacity in amperehours, so you'll need the current. The power supplied is constant, so the worst case current draw (highest) occurs when battery voltage is lowest. For a standard 12V battery it's nearly empty at 11V, so I'll use that.

$$I_{bat} = \frac{2.2W}{11V} = 0.2A$$

With that you can get the hours the battery will be able to supply the current:

$$\frac{100Ah}{0.2A} = 500 h \approx 21 days$$

It will probably run longer than that, as this is basically a worst case view on this, but there is some data missing to give a better estimate.

You should also be aware, that the output voltage ripple might be too high for your system to tolerate depending on how good that switch mode supply is.