If your roof was installed to manufacturer’s specifications and maintained properly, your roof will have a 20 - 30 year lifespan.
If the existing roof is in really poor shape, such as tabs being severely curled, granule loss, or bald areas, then complete tear off and replacement should be considered.
There is no known documented research, but most roofing professionals agree that with an overlay, the average lifespan of the shingles will be shortened by about 10%-20%.
Additionally, if your roof is not properly ventilated the life of your roof will be significantly affected. This is especially so in the hotter regions of the country. This will also affect your manufacturer’s warranty. The rule of thumb is 1 sq. ft. of ventilation for each 300 sq. ft. of attic space, evenly distributed between intake and exhaust ventilation.
If your roof has more than one layer of roofing, the roof should be torn off. In most cases, this is a code requirement; codes rarely permit more than two layers of roofing.
If you have multiple leaks, in the same or different areas it is time to have your roof thoroughly evaluated. Leaks may also show up in the form of dry rot on fascia, this is caused by water traveling under the roofing material and exiting at the fascia.
Spongy areas and sagging areas between the rafters or trusses are usually good indicators of damaged or deteriorated wood. If there is any suspicion of bad decking, a full tear off should be considered.