。
She kept him waiting for thirty minutes.
Literal
She [topic-は] him [object-を] thirty minutes made-wait.
待たせた is the causative-past of 待つ ('wait'), with the person made to wait marked by を. Note the use of Arabic numerals (３０分) rather than kanji (三十分) — modern Japanese mixes Arabic and kanji numerals freely; Arabic is especially common for specific quantities and times in casual writing. The duration phrase ３０分 sits adverbially before the verb, no preposition equivalent to English 'for' needed.