If it is frosting over after three or four weeks of use, that is normal. If it is frosting over a lot quicker than that, there could be an air leak where the freezer door gasket seals against the wall of the freezer. Also, frosting will be worse in humid climates, or when the freezer door is opened up frequently.
We've had to defrost ours once, and did it by loading up the freezer contents into an insulated cooler. Then we turned off the refrigerator, and poured hot water into pans on the freezer bottom. Change out the water when it is cool, and pretty quickly the frost will be gone.
I wouldn't defrost until the frost turns into a problem, like reducing cooling in the freezer or occupying so much room that the ice cream will not fit.
There's lots of advice and information in forums... sometimes it is correct. For example, all of my posts are made by a political appointee who got the job as a reward for contributions to my diesel bill.
2011 Jayco 28.5RLS; 2008 Chevy Duramax; Pullrite Superglide Hitch