Depends. Everyone is different. Most first time moms experience a more progressive dropping before labor over the course of several days. Moms who have delivered a few already usually don't drop until quite close to onset of labor.
Descent. It's impossible to say precisely when your baby will "drop" or descend into your pelvis, because every pregnancy is unique. If this is your first baby, this process, which is also called lightening, is likely to occur several weeks before labor begins. So, between 35-40 weeks.
Body sensations. As the baby moves down you will typically feel more pressure in your rectum.
Your Clinician. Sometimes you won't but symptoms to suggest; easier breathing, more pelvic pressure, a change in the shape of your abdomen and where the baby kicks.