Personally, when I drop the landing gear is a function of many factors. In this day and age of fuel conservation and high operating costs, I try to minimize the amount of time flying around with the gear down.

Operators have operating profiles that call for the landing gear at specific intervals. These often have allowances for crew preference or operating conditions such as a long final or speed restrictions.

Typically pilots will drop the gear after adding some degree of flaps, however the gear can also be used to slow the plane down (such as if it is heavy). Either way, the gear will need to be down by an arbitrary speed (in my aircraft it's 150 knots) or the gear warning horn will sound. The warning horn will also sound if full (landing) flaps are selected and the gear is not down.