So once i reached a certain level or time spent or something i gained a 100 coins,there isn't any specific guidelines or rules to this i could find
Anyone have an idea what does it actually take to gain them ?

Buy some more or win a coin game.

the 100 coins is just to let you get a feel for how coin games work.. when they first came out it was enough for 2 coin games, now it's enough for 20 if you play the cheapest game possible.