Be ware that a lot (all maybe) of the amazon car starter batteries quote a mAh thats not at 12v but instead at 3.7v which is the voltage of a lipo cell. (Probably jay car too)
for example this one
https://www.amazon.com.au/gp/product...3-d15c44e89a61
says 42 AH but 155Wh
you can tell because the wattHours is amp hours (42) x voltage == 155Wh
They probably have 4 cells to get >12v so you can divide the amp hours by 4. then maybe half it again if you are likely to average 2Amps of usage to find the true time the battery will last.
And thats assuming they arent inflating the numbers anyway.
Mike