That isn't paint coming off the sides, it's the fiberglass breaking down and the white is the residue.  I had an 84 Trailet and that was happening and I was told I could wax it with fiberglass wax.  But in the meantime, I had the stripes on the trailer painted over to match my truck.. and the roof, too.  I asked them not to paint the white part since that was part of the "coolness" of the roof, so they painted the radius and then clearcoated the whole top.. and that stopped the breakdown of the roof.  I don't know what type clearcoat they used, but it was like a body shop that worked on trailers.

Jbsny

As Jbsny stated, the roof coating isn't paint, it's a specialized polyester resin called a gell coat, that protects the fiberglass matrix from weather and UV damage. It is now damaged and this can be confirmed by wetting your finger and rubbing the surface. If there is any white powder transfer, the gell coat needs attention. .

The application of a marine paint is the least expensive, and easiest way to apply a new surface. Spraying a clear coating of polyester paint will temporarly seal the dead resin, but will do little to further protect it from UV damage. A white coloured paint will reflect the most amount of sunlight, and provide the best long term protectionIf it doesn't receive any, the surface will continue to detoriate, and the whole layup will degrade and weaken.

Waxing a damaged gell surface will only make the repair more difficult. The first order of business is to remove all of the "dead" resin. The easiest way is by sanding. When the powder has been removed, test the bare surface with a rag dipped in acetone. If there is any further resin transfer to the cloth, the roof will have to be recoated.

It depends on several factors. The thickness of the gel coating and the layup will depend on the manufacturer's construction techniques. The quality of the gel and laminate will depend on the materials he used.

The thicker and better the quality of the gel coating, the longer it will last. We used to coat with a 30 mm layer of Iso resins and had little UV problems. Once the gel is effectively degraded, has oxidized and washed off, the laminate is vulnerable to the same UV damage. This can be observed when the surface coating is no longer smooth, and a layer of fiberglass mat or cloth is exposed and visable.

 Unfortunately, laminating resin has little resistance to UV damage, and in weight saving measures and strength considerations, just enough is used to support the glass fiber matrix. It will be quickly damaged, and in doing so, will offer less and less support to the fibers. This results in an overall weakening of the glass panel, with a noticeable increase in flexation and possible cracking. Once this happens, the only recourse is to replace the panel, or over coat it with more glass and resin.

Your answer about time can't be easily given. It depends on where you live and how much exposure the trailer has to the sun. Is it stored under cover? Is it subject to freezing and thawing? Again, construction considerations vary. It may last only a few years with no damage or a decade or so with little problems.

It's easy to tell if there is any damage. Wet your finger and rub it on the fiberglass surface. If there is a powder transfer to your finger, it has suffered some UV damage. The amount that is removed and how much resin remains, is what will determine the roof's longevity.

Painting is the easiest and most effective way to stop any further damage if the structure is still strong. This is a common practise in the marine industry. The other restoration recourse is to apply more resin and possibly some laminate, sand the lot smooth and polish. This is expensive and labour intensive.

If you have a modicum of DIY abilities, painting your own trailer roof falls with that category. The only power tool you need is an electric sander. If I were farming the job out, I would choose the technician with the most experience.

Yes there are a multitude of marine products. I've tried several and found this paint to be the best all around for my needs:

http://www.jamestowndistributors.com/userportal/show_product.do?pid=65&familyName=Interlux+-+Toplac

There are less expensive products that also work well. These are my favourites:

http://www.jamestowndistributors.com/userportal/show_product.do?pid=64&familyName=Interlux+-+Brightside+Polyurethane

http://www.jamestowndistributors.com/userportal/show_product.do?pid=1329&familyName=Pettit+EasyPoxy+Topside+Paint