There are a lot of variables to this answer and there is no hard fast rule. The first step is to identify what type of shingle you have. The 2 most common types of shingles are 3 tab and Architectural. In general a 3 tab roof will last somewhere around 15 years. An Architectural will typically last around 20-25 years. (See the next sections for identifying your roof) There are many variables that will cause a roof to either deteriorate faster and last longer. Attic ventilation, installation, sun vs shade and quality of substrate just to name a few. If your roof is over 15 years old, an onsite inspection is recommended to assess your roof condition.
A 3 tab roof will look like the above picture. It has a very flat and plain appearance. It will typically start to show staining on shady areas or on the northern facing slope after just a few years. You will be able to see imperfections in the decking very easily. They are very susceptible to wind and hail damage.
An Architectural shingle, pictured above, will have a more 3 dimensional look. There will not be any "tabs". They are generally more appealing, more resistant to staining, and will perform better than a 3 tab in every scenario if installed correctly.
How important is your warranty to you? What most roofers trying to sell you a roof with the greatest warranty since sliced bread is that lack of ventilation or an improper ventilation set up will cause your manufacturer warranty to be voided as soon as your new roof is installed. An attic inspection by your roof estimator is required to determine if you have a balanced airflow system.