、。
Take the 50-minute mock exam, and you'll see just how strong your skills are in real testing conditions.
Literal
50-minute [genitive-の] mock-exam [to-に] challenge-and, exam [at-で] your [genitive-の] ability [subject-が] how-much [is-か][subject-が] is-understandable.
A product-pitch-style sentence. 挑戦する ('take on, challenge') takes the object of the challenge with に rather than を — the challenge goes 'against' the target. The main clause embeds an indirect question: どれくらいか ('how much is it') is a wh-clause with the か marking it as a question, and then が makes the whole clause the subject of 分かる ('becomes understandable'). So the structure is: '[how-much-your-ability-is] becomes clear.' 実力 literally 'real strength' is a standard word for 'actual ability' as opposed to claimed or potential ability.