Quote from dagnyt:
The longer you hold the greater your potential return - unless
a) your short strike is threatened
or b) the market runs away from your strikes. This is especially problematic if you paid a debit for the position.
Thus, with diagonals, your intention should be to hold and let time do its thing.
But remember, although theta is your daily ally, it's VEGA that will determine the size of your payday. It's VEGA that tells you how much you collect when you eventually close the position. This is especially true when youa re able to hold until very enar expiration day.
IMHO, if you get a nice sized spike in VEGA, think about taking the nice profit (if it's available) and close. If your short strike is threated and you still have a profit, that's another time to consider closing.
If your short strike is breached - do not pass go, do not waste time - take action and close (or adjust) the position. You might want to open another position at that time, but don't take chances when the short strike is breached.
Mark