Even in the long term, I am skeptical that the male sex drive wanes much (and if does, it is age- or health-related, not a product of over-activity). I have met too many horny old men to believe that after middle age, a man's desire to impregnate females is diminished in any significant way. I am almost entirely sure that this idea was conjured up because the thought of grandpa eye-fucking his granddaughter's friends was too unsettling to be stomached by the general public.
A man can significantly reduce his sex drive by abstaining from sexual activity for about three weeks. I know this because I have done the infamous "thirty-day challenge" twice now, and have experienced it first hand. There is also a good deal of empirical evidence that suggests twenty-one days as the time it takes for the human brain to adjust to just about any routine (i.e. to make or break a habit). Much like the appetite for food in a person who fasts for a long time, a man's sexual appetite will shift into starvation mode after being continually denied. Although his initial desire for an orgasm will not fade much, he will be abnormally disinterested in sex once it is satisfied the first time after the three-week abstinence. This reduced libido will last until he slowly gets back into the swing of things by having orgasms more and more frequently. But even in an extreme case like this, the rebound does eventually occur, and before too long the man in question will be back wanting to fuck everything and anything.
So if your husband or long-term boyfriend gradually stops wanting to have sex with you, it does not mean he isn't horny any more or that his sex drive is starting to diminish; it means he is getting off in some other manner. Probably he is just masturbating to pornography, but the amount of self-discipline it would take for him to abstain from sexual pleasure altogether is just too great for it to be even a remote possibility without some hugely powerful force motivating him - like pride, intense shame or extreme religious conviction.
The plus side, which is the point I want to make here, is that all you need to do to keep your man sexually interested in you is to remain his one and only outlet for sexual activity; the sheer power of his sex drive and its persistent regenerative qualities will make him always want to fuck you, even if (eventually) the frequently slows slightly. How you remain his "sole source" is the topic for another post. Right now I just want to make the point that as long as you hold that role, you do not need to worry about his sex drive slowly tapering off any more than you need to worry about his hunger for food gradually disappearing. These things simply do not happen.