She apparently joins as one of the lower ranks - ranjen, prylar, not sure of the difference really - but by the time of RBoE has already become a full vedek. When Sisko questions the speed of such a progression, she says that she thinks the Assembly took her time as the friend and assistant of the Emissary into account as "time already served."
A vedek? I know Kira is religious, but it struck me that she fancied a family some time. I'm thinking of that time when she was babysitting the O'Brien's son and she said something about wanting a child of her own.