RobsSanta wrote:

My brain has totally gone blank for this simple question, I know I should
have paid more attention in school !!!.

I just was given a 12volt 1200mAh battery and a 200mh 3W wall charger ...
So the obvious question is, how long should it take to full charge this
battery using the supplied wall charger ?

Assuming the charger is actually 200 mA (milliamps) then if things were
100% efficient it would take 1200 mA-hr/200 mA = 6 hr to fully charge.
But allow an extra 25% or so for inefficiencies and about 7 - 8 hours
should do if it starts out fully discharged.