Mirena can stay in place for five years, after which it will need to be replaced. Women who decide they want to get pregnant can have the device removed at any time by their doctor. The official Mirena website states that women can start trying to conceive immediately following the removal of the device. They add that eight out of ten women succeed at getting pregnant within a year of removal. This is similar to the general pregnancy rate for any couple trying to conceive. Some message boards, however, tell a slightly different story.
Google "pregnancy after Mirena removal", for instance, and you will see page after page of discussions on this very topic. Some women report not having a period for quite a while after the removal of their Mirena coil, and others say their doctors told them to wait a couple of months, until a regular cycle is established again. Some doctors also point out that the uterine lining may be too think to sustain a pregnancy right after removal, and add that waiting a while may be a good approach.
On the other hand, there are also plenty of women who say that they had a positive pregnancy test very soon after getting their Mirena coil taken out. It is even possible to catch the first ovulation after Mirena, without ever having had a period! Before making a decision, it is good to discuss the implications of trying for a baby right after Mirena with your doctor.