？
At that rate, you'll never climb higher than the encouragement prize, you know?
Literal
Like-that [if-んじゃ] however-long pass-[even-ても] encouragement-prize stopping is [ぞ]?
A gruff critique. そんなんじゃ is a casual contraction of そんなのでは ('if it's like that / at that rate'). いつまで経っても ('no matter how much time passes / however long') uses まで + 経つ + ても for an idiomatic 'forever, no matter how long.' 止まり ('stopping at') is a noun form of 止まる used as a suffix meaning 'capped at, limited to.' 奨励賞 ('encouragement prize') is the consolation category in Japanese art/literary competitions — the honorable mention. だぞ with sentence-final ぞ gives masculine assertive force.