Its time to peek when:

• The cameras show that there is a SINGLE Enemy and he is facing the other way
  
• You are a Caviera and know that your target is getting revived
  
• There is one enemy left and you have AT LEAST 2 more friendlies peeking with you
  
• You need information and can afford to peek for a quarter of a second
  
• There is a downed/low health enemy that is in the open, and you can shoiot before getting lit up
  
• A friendly needs a bit of cover fire. In this situation, prefire the hell out of the hallway!
  
• If you have a shotgun and you know they are right by the door.
  
• You are new, at least one of these is true, and your team is able to handle your death

• If an enemy is downed and you know there is someone watching him. He may be baiting you.
  
• If a friendly is downed and there are known enemies around him, he is definitely bait!
  
• If there is more than one person in the hall and they are watching your entrance
  
• If you are a newer player and have a hard enough time dealing with recoil during easy shots
  
• If you are playing ranked and are not absolutely sure of your capabilities as a player
  
• If you have bad reflexes
  
• If you rarely get headshots
  
• If you dont understand the "ghost separation' mechanic
  
• If your character is 1 speed 3 armour and they know you're there
  
• If your enemies have already shot someone out of that door/window
  
• If there are multiple available angles to see through that door/window
  
• If you have a shotgun and they are far enough away that it will take more thn a couple shots.
  
• If your primary has a sight other than an ACOG or Red Dot
  
• If your other team is full recruits. Trust me, its a pro recruit team. Always. Every. ♥♥♥♥♥♥♥. Time.
  
• If you have bad aim and arent afraid to admit it
  
• If you have bad aim and are a stuckup ♥♥♥♥

If you absolutely need to peek, remember that your ghost is what they shoot. Whenever a player moves, the other player sees a 'ghost' that is at least a few milliseconds behind where the player currently is. If you can peek fast enough, by the time the enemies light up what they see on your screen, you could be safely in cover again. This is the reason some people will purposefully raise their ping to close to 150-200. It also helps their opponents, but they know it buffs them.