It’s no longer a question of whether China’s stock-market rally is a bubble, but when the bubble will burst. A market crash may come within six months, Bocom International Holdings Co. said Tuesday, citing an analysis of global bubbles over 800 years that shows the speed of gains in China mirroring past market peaks. Macquarie Investment Management, whose Asian stock fund is outperforming 97 percent of peers in 2015, has already eliminated exposure to mainland shares after turning bearish for the first time in seven years.