Of course if you do what is right and maintain your roof right, it will last as long as the roofing manufacturer warrants. But the disclaimer is that the shingles will only last as long as they are suppose to last with proper maintenance.
So what is proper maintenance? As a contractor who has been in the business for over 15 years, I will break it down. First of all, if you can have your roof treated once per year with a good organic roof treatment by a reputable company who is fully legal and knows what they are doing then you should be fine as this will deter any growth. If you wait too long and you do get some growth, each time you have it cleaned you are losing life. So the key is to have it treated at least once per year by someone who understands how to walk safely on a roof and minimize foot traffic. Our method is from the ridge line and from the gutter line from ladders to minimize foot traffic, while still getting a good soaking on all parts. We use a registered organic with the OMRI label, so it is safe to use around fish, waterways, plants, gardens, grass, etc. Most other companies use sulfates such as zinc, which is very caustic and hard on our water ways.
Another thing that you should have done is have regular inspections for leak hazards. We inspect, repair and we fix! Emails with photos and a detailed inspection about what needs to be done. The main things that we see are exposed top nails and worn out pipe boots. Both of these are leak hazards. Pipe boots wear out about every 5-10 years so it is important to have these replaced when needed or they will cause more problems later when they start leaking.
We get people on a twice a year email reminder list for the treatment, but only expect to do the treatment one time per year. A lot of people like us to clean their gutters at the same time.
This job here in the listing photo is a great example of one that is an older roof towards the end of its life, yet we can still help. We use a soft touch and only remove the amount of moss that will be beneficial for the roof followed by a treatment and hopefully a yearly commitment to treat it once per year. See as long as you have your roof treated at least once per year, the minimal amount of moss that you have, will take care of itself and be less and less noticeable.
A lot of times, we will treat first, then come back and do the removal. This is because dead moss comes off of a roof a lot easier than live moss. So many times, this method is best for the shingles. We only want to do right by you and get you the longest shingle/roof life possible.