How do I calculate battery life for my devices?
The current rating for the converters is only the maximum current it can deliver, thus is not useful to calculate the power drainage.
To get the time you have to know the real input current for every DC converter(\$I_i\$) then, since they are in parallel,

$$I_{tot}=\sum_i{I_i}$$

To get the time, suppose \$Q_p\$ is the capacity in Amp/h of your battery, \$T\$ is approximately the time it will last (in hours)

$$T=Q_p/I_{tot}$$

So, if you can measure the \$ I_{tot} \$, you can directly use the aforementioned formula.
Keep in mind you should not completely discharge a lead battery or it will decrease sensibly the capacity.