I have been playing my games with two main phases in which I am able to cast creatures and stuff. However I just watched a spell slingers episode and one player decided to make a load of actions at the end of his opponents turn.
How far can you exploit that end turn of your opponents?!
As long as it's at instant speed, you may cast it when you receive priority. That includes the End Step, or end of turn. So yes, they could have replied by casting multiple spells at the end of their opponents end step
Very far actually! If you run instants like {Sphinx Revelation} you can draw a lot of cards and gain a lot of life!
If you run creature with Flash like {Vendillion Clique} you can cast it and see their hand so you know what they can do on your turn :P
In standard they can do {quicken} and play any sorcery during your turn