You can only do so in your turn. You may do so during Main Phase like a normal Spell Card. You can also do so to start a Chain. For example, if your monster attacked and your opponent activated Mirror Force, you can Chain My Body as a Shield from your hand. If you plan to activate a Quick-play Spell Card during your opponent's turn, however, you must Set your Quick-play Spell Card during your Main Phase.
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You do not have to Set a Quickplay Spell Card to use it in your own turn - just the same as you do not have to Set your regular Normal Spell Cards before you activate them. You can chain a Quickplay spell from hand too, as they are spell speed 2 - that is, you don't have to set them if you want to chain them to something. So if it is your battle phase and you attack, and the opponent uses Mirror Force, you can chain My Body as a Shield from your hand.However if you want to use them in the opponent's turn, you do have to set them on the field.
In your own standby phase, yeah. It is a Quickplay spell, so can be played from hand in your own turn. You can draw it, then activate it in the Standby phase, tributing Treeborn Frog, then resummon the frog before the standby phase ends.
There is no real hard limit - as long as you've got a spell card in hand, and a slot in the S/T zone for it to go, then you can activate it.
no you have to have a field spell in your deck otherwise it would say you may or you can add a field spell to your hand from your deck You are obliged to know the content of your own deck, and know what cards are left in it (by looking at what has been drawn already). You cannot activate an effect that tries to search for a card that you know it will not be able to find.
regular spell cards can be activated only on your turn, quick play spell cards can be set then activated during your opponents or your turn, and spell cards with the infinity sign are treated like regular spell cards but stay on the field until it is destroyed traps cannot be activated from you hand they must first be set then activated on your next turn or your opponents turn knowing when to activate these spell or trap cards is tricky you have to know when to activate them or they will end up not helping you out at all