3 tab roofing is a single sheet of asphalt material about 3 feet wide with 2 vertical tabs cut out at the bottom which results in 3 tabs similar to tabs on a file folder.
I recommend inspecting 3 tab roofing every year or two for weathering and wear.
When you first buy new 3 tab asphalt shingles, they are unbroken, fully and evenly coated with granules and flexible. They also stink when placed in the sun. The asphalt in the shingle helps repel water. The granules absorb the suns UV rays to protect the asphalt oils. And embeded within the shingle is typically fiberglass to help it retain its dimensions. Please remember these about new asphalt 3 tab shingles: evenly coated with granules and flexible.
As the sun's ultraviolet rays (UV rays) beat on the shingle and raise its temperature, the volatile oils evaporate out of the shingle. It is the oils in the asphalt that hold the granules on the face of the shingle. The granules protect the asphalt tar and oils from the sun's UV light. When granules drop off, this allows the sun to hit the asphalt directly. This "alligators" the asphalt/tar in the shingle and kinda fossilizes it. This make crystals form in the tar. As crystals form in the tar, the shingles (if you were to try to flex them) would become stiffer (less flexible) and as you flexed it it would crack (and it does). You may not be able to see the cracks, but you can definately feel the brittleness of the material as it ages. Wow! The sun's UV rays are brutal. You want to make sure the granules don't fall off your shingle too rapidly. Granules are too asphalt 3 tab roofing as paint is to a car. If you appreciate and take care not to damgage these granules, your roof will last longer.
So the big question is "When should I replace my 3 tab roof?" The obvious answer is "You should replace your roof right before it would start leaking." So the next question is "When does it start leaking?" Let's think back about the new roofing material: evenly coated with granules and flexible. So if you check out your shingles (sunny side weathers fastest), look at them closely. How many granules are gone? Can you see pockmarks in the shingles where granules have popped out? Are there waves of granules missing where someone pressure washed (horrors) the granules away in waves? Can you see cracking (allowing leaking) in your shingles? When you grab a lower corner of the shingle and try to bend it, is it stiff and brittle, or does it flex fairly well?
Now, before you run off and buy a new bundle of shingles to replace your roof, please consider these things. Underneath each tab is the top side of the lower shingle that has not been exposed to weathering. So whatever gets through the top tab, would probably run out down below. So while the top tab of your shingle may leak, the one below it will carry the water away.
Missing shingles can be removed and replaced. As your roof ages, it becomes harder to remove and replace individual shingles without adjacent ones being broken or cracked
I have seen steep roofs with shingles that had tabs that had 50 percent or more of granules gone, cupped and curled and the roof never leaked last I heard. It was also under some trees in part and was probably 50 years old. Not saying you should let yours go that far, but just so you know what I've seen before. It may be for cosmetics that you replace your roof before it leaks.
Financially, you may calculate the cost of a new roof (call 10 contractors, maybe 5 will give you bids and pick the middle bid) and divide it out over the life expectancy. Now figure out how much a roof leak would cost. How is the timing in your life? What is the roofing marking doing? If the economoy is slow, you have the cash (or going to do it yourself) you may want to get the roof down now. It is probably going to be more expensive if a lot of houses are being reroofed.
If you are in a housing development, and your roof needs replaced, chances are that other roofs in your area are going to need replaced also. You may be able to work with some other homeowners as you shop around to get a pricing discount.