Welcome to the forum! There are a lot of variables when it comes to longevity of a product. Is your car garaged, how often do you wash it, where does it sit while at work etc. I have found that Americana will last about 2+ months on our daily driver. I now use H2O GG on it, a Jeep Wrangler. H2O is a fantastic product. I just went 3+ months with an application and it still beaded like the day I put it on. I use the H2O not only on the paint but all the black plastic, headlights, taillights, and wheels. For your BMW you could use the H20 next time then, when needed, top that with the Americana.
Another consideration, since it was 80 today in Central PA, is bugs. I do not know where you live, but using the H20 on the front of your BMW will help remove those bugs. I do not wipe our Jeep down every time we drive it but I do when it comes to our other cars. I have a sprayer with diluted rinseless wash which makes bug removal easy. You have the waterless wash which works just as well. As far as washing goes that is up to you. Once a week is great if you have the time. The important factor is getting a solid routine down to prevent scratches while washing/drying.
Post some pictures of your car!