The first primary teeth to fall out are usually the lower front two (mandibular central incisors, usually at the age of 6-7 years old). This is usually followed by the upper front two (maxillary central incisors). The reason primary teeth fall out is because the permanent tooth usually resorbs (removes) the root of the primary tooth as the permanent one erupts. This happens unless something else causes the primary tooth to come out like trauma. Once the primary tooth has fallen out due to the permanent tooth resorbing the root, it should take a couple of weeks to start seeing the permanent tooth. Remember the primary tooth falls out because there is not enough root to anchor it in; however, the permanent tooth still has to go through the gum tissues to be seen. Hope this helps.
My best to you!
William F. Scott IV, DMD
It varies by person. It can take up to several months.
Thank you for your question.