Having a baby is one of the miracles a couple can face in their life. Still, in the beginning it can put an end to even the most balanced and harmonious sex life. How long this period takes depends on various factors, but what you need to know is that if you are lucky, she is ready to have sex again within a couple of weeks. Unfortunately, though it can take several months, as well, especially if postpartum depression occurs, as well.
The sooner the woman can start the Kegel exercises (that strengthen her pelvic muscles, so her vagina can turn tight again, and that can be practiced the best if she stops peeing, as she uses those very muscles for that purpose, as well), the sooner she will be ready to have sex with you like she was before pregnancy.
You need to keep in mind, though, that her vagina will need proper lubrication for a while, so never forget to buy one. Since her vagina is still sensitive to infections, do not use body lotion as a lubricant these days.
And last, but not least, let her take a nap, or wake up a couple of times a night instead of her when the baby starts crying. This way, she can have at least a couple of hours of sleep, so she will not be tired as hell when you get to bed together.
If you pay attention to these simple rules, then no baby can ruin your sex life ever.