How long can they keep an inmate in the SHU prior to transfer?

Do you know why he is in the SHU? The reason behind being in solitary would probably answer why it's taking so long. When I was in the SHU, there was a guy there for 11 months awaiting a transfer. He was caught with a cellphone and had to do 18 months in the SHU before they would transfer him to a higher custody prison. Administrative makes me think that he might have asked to be taken out of the general population for some reason. If that is the case, then he will have to wait until they find a suitable prison to ship him to.