At 50 my ideal frequency for shared sex¹ would be 10-12x a week, inclusive of masturbating myself at around 2-3x a week.
While the current reality for me with my almost 51 year old wife, for shared sex is most often at a frequency of 6-9x a week (with 4-6x of that being on the weekend), with masturbation for me being at around 3-5x a week (mostly on weekdays).
Although there are some weeks, mostly when my wife has her usual monthly (for the moment fortnightly) RDO, or when we are on holidays. When we still manage a shared sex frequency of 11-14x a week, with masturbation for me being at around 1-2x a week.
At the lower end when we have a week that is limited through the demands of other external activities. We share sex at 4-6x a week, with my self masturbating at 4-8x a week.
There was also a significant dip to around 4-7x a week of shared sex (and I was fine with that), when we were in our early thirties, As a consequence of work demands and having young kids as well. Yet our late 30s saw things improve significantly.
That said I have no doubt my ideal frequency will decrease over time, fortunately I haven't had any sexual problems so far. Since when I was in my late teens through my twenties, my ideal was 14-18x a week of shared sex. And I usually had that frequency and occasionally even higher, when in ongoing sexual relationships. Including with my wife, ex-wife and a few others back then.
One thing for sure at this age I don't think I would settle for a shared sex frequency, of 3x a week or less as the norm with my wife. Without ending our relationship and seeking sex elsewhere, or carrying on with her and seeking sex elsewhere to try to meet the shortfall.
I figure if I live past 70, I will be doing well, to enjoy some form of shared sex at 2-3x a week after that. Of which the fact that I had grandparents who were still sharing sex in their 80s, tells me that it is possible.
Also since
@badsanta mentioned time taken. Morning sex for us tends to be around 15-30 minutes, daytime and evening sex at home or in accommodation tends to be around 40-80 minutes, while outdoor sex as opportunity presents (with no-one around) tends to be at around 5-15 minutes.
¹
Shared sex for my wife and I mostly sees us at least share penis in vagina sex, plus penis in rectum sex and oral sex as well. While standalone oral sex for either of us is fairly infrequent (since it usually turns into all of the rest). Although it does happen from time to time, and is usually done on request.