As a fresh recruit I am curious how often folks believe razors should be sharpened (as opposed to stropping)
The simple answer is when the edge begins to tug or when you feel like touching the edge up. And you must strop between each shave.
The long answer is there are multiple factors, including the quality of the bevel, the steel and the bevel angle.
When I started, I needed to touch up my edges every 3 to 5 shaves. Now my edges last much longer - normally over 10 shaves. I assume this is the result of my honing improving over time.
I should add that I often touch up edges before it is required simply because I enjoy honing and trying different edges. And this is one reason why I have mainly bought user-grade razors.