It depends on what type of bougainvillea you have & what shape you want it to take on. Mine which grew up & over my garage in Santa Barbara was Bougainvillea glabra & it reaches 20'+ tall. I pruned it down to 2 major trunks & trained it to grow up the right side of the garage. I attached a metal trellis to the center top of the garage & trained it to grow across the entire span. It was work, but very well worth it in the end. My Bougainvillea Barbara Karst (which reaches 25') next to my garage here in Tucson never gets that tall because a freeze usually knocks it back. When I moved into this house, it had been pruned as a 5' hedge & never flowered. I let it grow out, opened it up, & now it grows in an open shrub form. It blooms on the regular which is what we all want. And, I keep it under 8' tall so it's easier to prune.