It is common for both husband and wife to undergo changes in libido as they get older. Halachically, once a week (if possible, on Friday nights) is generally considered a minimum — unless husband and wife both agree to less. While there may be some encouragement for a couple not to have relations constantly, there is no clear maximum.
Since this concern is less of a halachic issue, it is likely best addressed by speaking with your spouse to come to agreement about what would best meet both of your needs. Sometimes less quantity may be balanced by more quality. Or it may be that there are steps you can each take to increase your interest in relations.
In some cases, when a couple cannot reach a mutually satisfactory arrangement on their own, counseling can prove helpful.
Please let us know if we can be of further assistance.