。
She kept me standing for as long as two hours.
Literal
She [topic-は] me [object-を] 2-hours [as much as-も] caused-to-stand [left in that state-ておきました].
Three grammar layers stack here. (1) The causative 立たせる ('made stand') from 立つ ('to stand'). (2) The focus particle も attached to a quantity (２時間も) emphasizes that the amount is surprisingly large — 'as long as two hours,' 'a whopping two hours.' (3) The auxiliary ~ておく ('do in advance / leave in a state') paired with a causative becomes 'leave [someone] doing X' — making the standing not just a momentary state but a sustained, allowed-to-continue one. The polite ~ました raises the register, fitting a complaint to someone who wasn't there to witness it.