Repair and replacement
Treatments work on the surface. They do not rebond layers or close splits. That single distinction decides almost every repair-or-replace question we get asked, and knowing which side of it your roof sits on is worth doing before spending anything on treatments.
Structural damage means the fabric itself has failed: splits along the fold lines where the roof creases every time it is lowered, fraying at the edges, or the inner headlining delaminating from the backing. None of that responds to cleaning or proofing, because none of it is a surface problem. Minor tears and a rear window that has let go are both genuinely repairable, and rebonding a glass rear window is a routine job rather than a reason to replace a whole roof. Beyond a certain size of split, though, replacement is the honest answer.
Faded fabric is the other case where expectations need setting first. Colour restoration works well on a roof that has weathered gradually and is structurally sound; the fibres are fine and the surface colour has dulled. It works far less well on fabric that has genuinely bleached, which is what happens to dark roofs after years of strong sunlight. Treated, a bleached roof can come out patchy rather than uniform, which looks worse than the even fade did.
The case people most often assume is terminal usually is not. A green roof almost never needs replacing. Algae, moss and lichen are cosmetic contamination sitting on the fabric, not damage to it, and they respond well to a biocide treatment and patience. Lichen in particular does not lift on the first pass, which is why owners conclude the roof is beyond saving when it simply needs a second treatment.
How long a hood should last is not really a number. A roof that is cleaned before contamination embeds, proofed when it stops beading, and kept off pressure washers can last the life of the car.