Age and remaining life
A three tab shingle rated for 20 to 25 years or an architectural shingle rated for 30 or more gives a rough baseline, but actual condition matters more than the manufacturer's number. A 15 year old roof with heavy granule loss on the sun exposed slopes has less real life left than the rating suggests, while a well ventilated 20 year old roof on a shaded lot can still have years in it.
Granule loss, visible in the gutters or as bald patches on the shingle surface, is the clearest visual sign that a shingle is nearing the end of its protective life regardless of its age on paper.
Number of existing layers
A roof with two layers of shingles already on it cannot legally take a third in most jurisdictions, which removes the option of a simple overlay and points toward a tear off regardless of the roof's overall condition. Checking layer count is one of the first things done on any inspection for exactly this reason.
How many separate leak points exist
One leak traced to one bad valley or one cracked pipe boot is a repair. Leaks showing up in more than one part of the house, especially on different slopes of the roof, usually point to a shingle that has failed broadly rather than a single isolated defect, which shifts the recommendation toward replacement.
The decking condition underneath also factors in. Soft or delaminated plywood found during a repair often means the leak has been active longer than assumed, and repairing shingles over compromised decking just delays the larger job.
Cost over time, not just up front
A repair costs less today but does not extend the roof's overall life much if the shingle is already near the end of its rated span. Spending on a repair for a roof that needs replacement within two or three years is usually money that would have gone further put toward the replacement directly, which is why the honest recommendation sometimes costs more up front.