You should upgrade them at the latest possible opportunity, the higher your personal Attack / Defence for this, the better. Their stats are directly linked to your personal strength, so there is no need to be upgrading them for a good long while. I am lvl 648 and i still have some defenders with around 180 defence lol, that i bought very early on in the game. The simple answer is when your Top Squad Strength is no longer 20 x your own personal strength (when you are no longer capped) Fortunately having very strong players in your top squad can keep you capped for a considerable amount of time, but that gets a bit deep and complicated with excess strength filling in the gaps of any weaker slayers to keep your Top Squad Strength at 20 x your own.
