As a first approximation.
Those batteries are rated at 5Ah (more on this later).
At full power a 12V 250W motor will draw about 21A so on the face of it one battery will give you about 15 minutes of operation at full power.
However standard lead acid vehicle batteries don't really like being used by this, they are designed to give short bursts of high current for starting and then be slowly recharged so a constant load doesn't do them much good nor does fully discharging them so you are never going to be able to use the full capacity and ideally you want batteries specifically deigned for deep cycle use.
In terms of estimating power requirement you are probably better off looking at data of the power output of cyclists rather than trying to estimate the actual loads on the bike. A professional cyclist can generate something like 200-300 Watts for hours on end so something like 150W+ is probably a reasonable working estimate for casual cycling.
Of course as you add weight you will also need more power, especially in stop/start riding and climbing hills, at a constant speed on the flat weight makes much less difference.
A lot of powered bicycles have a motor rated for the average expected load which is supplemented by pedal power for accelerating and climbing.
You may be able to extend the range a bit with a regenerative braking system but again that comes with a cost of added mass.
Estimating range is more difficult but this calculator may help obviously that is aimed at competitive cycling but it is at least real world data.