Literally — Сколько (for how long) [уже] можно (is it acceptable) болтаться сзади (to lag behind)?
(with the implication that the amount of time for which it is permissible to lag behind has long elapsed)
This construct is a question that could be literal or rhetoric, only the latter sounds accusing. A child could ask a parent, “Сколько [мне] [уже] можно играть?” — “For how long may I play?” (“уже” would merely suggest that something has changed and the speaker expects a different allowance). However, if the parent were to say exactly the same (without “мне” obviously, or with “тебе”), that would be accusing the child of playing too much (and “уже” would play an emphasizing role).
Like plain “можно”, it uses the dative case for the subject (something permits (whom?)): “Мне можно [есть] шоколад, но нельзя мёд” — “I can eat chocolate, but not honey” (presumably due to allergy).