When should I resign?

I think that when one side is up a clean piece or more (no material or positional compensation for the other side) then the losing player should resign.

For the OP,

Usually I resign(in a classical game) when my opponent is up a piece and 1 pawn.

I do it in this fasion because in theory an extra piece is not winning as a K vs K and Knight is a draw to insufficient material. So my opponent must have an extra pawn, though it would be ideal if him or her had a pawn that was a b,c,d,e,f or g pawn, as the a and h pawns are harder to promote, and with the wrong squared bishop endgame that could also become a draw.

But also it depends on the position as well... Having a significant positional advantage matters way more than the loss of material(or in this case more material for my opponent). To put this in perespective, there are plenty of positions someone could be up material, however what good is it to be up material, when they will be checkmated in 1 or 2 moves?

So really the answer would be is when I am down a piece and a pawn or more and I am in an inferior position with no counterplay.

There are some exceptions, but for the most part this is the time to resign, for me at least.

...

yup, lost material and no counterplay.

In a recent game, I was up two knights (+6) in material after move 23.  My opponent didn't resign until after move 43 when I was up +24 in material.  And honestly it didn't bother me one bit.  I figure he wanted the practice and it was good for me to keep my concentration and avoid making some stupid blunder(s).  Ironically, when he did finally resign, there was actually a move I could have made on move 44 that would have ended up in a stalemate!  

Strong players know when to resign.  The stronger the player--the quicker he will resign against equal competition.

I have had situations where my strong opponent resigned in a position that 99.8% of the players would not resign.

@ponz111 

I do agree with what you are saying to some degree, the stronger will mostlikely resign earlier. This is usually because of experience and knowing a position is lost long before the positon unfolds because the stronger player has deeper understanding overall in the game of chess and due to this he/she resigns the game because they know it is lost somehow. Perhaps it could be lost because of a tactic, or it is just positionally worse, there a multitude of possiblilities in this instance.

But still, there are always exceptions as you never know what could happen in a game as, there could be counterplay or winning chances for an opponent. But as long as your position is not completley lost then you have a chance to have a draw or win in a position, of course depending on the position at hand.

So like a lot of questions in chess, it depends on the position and also some other varying factors...

@ponz111 I do agree with your initial statement, I was just adding on some more info.

Always resign when down by two or more pawns. Think about it this way: if you are up 2 pawns and you trade everything else, you end up with two pawns vs lone king, which is always a win(unless they are doubled rook pawns). You can often resign when down by a pawn. For example, when I was playing at my local chess club, I was "only" down a pawn, but I resigned. My opponent said "It's only a pawn. That's nothing". His friends also criticized me. However,I couldn't move anything because my queen and rook were tied down to blocking the pawn(It was a passer on the 6th). My king was also very unsafe and he had threats of discovery. He also had 2 bishops(I had a bishop and knight)