According to BGI, a bougainvillea should be pruned when its branches become so long that they are detracting from the overall shape of the plant or damaging its structural integrity. Depending on the size of the plant, pruning can involve removing branches that are several inches to several feet long. Use clean, sharp pruning shears to cut unwanted branches at a slight angle when pruning.
Because bloom initiation does not depend on trimming, BGI recommends pruning a bougainvillea at any time of the year. However, for the best flowering, it recommends pruning at the end of each blooming cycle or before the subsequent cycle begins. Because bougainvillea is a vine, the main vine continues to grow outward without side shoots unless its growing tip is pinched off or pruned, so it is important to do this if a fuller plant is desired.
BGI suggests avoiding waiting to prune a bougainvillea until its branches grow beyond the desired size, as this can create a plant that is excessively bare in the middle. Instead, use shears or the hands to pinch off all of its growing tips before it reaches the desired length to encourage it to fill in and look more evenly balanced.