。
She kept me waiting for 30 minutes.
Literal
She [topic-は] me [object-を] 30-minutes caused-to-wait.
Causative 待たせる ('made wait') with a duration expression as a bare adverb — ３０分 here functions as an adverbial of duration without any particle, a standard pattern for time-spans (cf. 二時間勉強する 'study for two hours,' 三日泊まる 'stay for three days'). Japanese uses Arabic numerals ３０ here in mixed-script style; 三十分 (full kanji) would be more formal, 30分 (half-width Arabic) more digital. The choice between numeral conventions adjusts register slightly without changing meaning.