It is possible for Depo-Provera to affect a woman's period as soon as the first shot, according to Planned Parenthood. The shot can result in an irregular or lighter period, or the woman's period can go away altogether.
WebMD says that up to 50 percent of women stop having their period within one year on Depo-Provera. Most women experience lighter or shorter periods even if their period does not stop altogether. The longer a woman uses Depo-Provera, the more likely it is that she stops getting her period completely. Most women experience irregular periods within the first six months on Depo-Provera.