I usually advise couples to wait until after the withdrawal bleed and two further normal periods (i.e. three months) before they try for another cycle. Three months is really a rule of thumb but going through IVF is a psychological stress to which a couple should not be subjected too frequently. Also, the ovaries often have reasonably large cysts after treatment, which need to settle.
Pregnancy rates become slightly lower for all forms of fertility treatment as the number of attempts increases. This is because couples have different chances of success and the couples with the best chances usually get pregnant first. If high quality embryos are being formed, the chances of pregnancy remain high.