When ever I feel like it. If she's in the mood, and I am too, then that too.
I rarely want to get busy whenever I'm too tired from my day, but there are many other factors that get in the way (namely age and physical activity outside of work). There really can't be a specific amount one couple should follow given that.
I've been getting back into PE (penis enhancement) which I've done in and off for a few years now and that has made me want to have sex more often along with nofap as well. Making my wife work out and noticing her progress has also made me want to fuck more, too. A lot of stuff goes into answering your question!