The windmill took longer than expected for two reasons.
First, there was a huge storm that damaged the farm. According to chapter six, that November the windmill was half-finished. But when a big storm took place, it destroyed the windmill. What made this situation even more interesting is that Napoleon declared that it was Snowball who skulked into the farm and sabotaged the windmill. This was a preposterous claim.
At this point, Napoleon offers an impassioned speech and pronounces a death sentence on Snowball. By doing so, he is able to rally the other animals together.
Second, when the humans saw the progress on the farm, they decided that something must be done. So, they came into the farm, and they decided to target the windmill. They blew it up. This caused another huge delay. According to the text, the month was August.