How often is it legal for me to have my boyfriend stay over in my rented apartment.

The answer to your question will be governed by your lease terms, or if you are a tenancy at will, the terms of your other tenancy agreement and/or "house rules" at the property, as long as those were sufficiently communicated to you so as to form a basis of your agreeent. Many leases prohibit a particular visitor from staying overnight in the tenant's apartment for longer than a specified number of days per year; it's common to see restrictions on visitation for longer than 14 or 21 calendar days in a year. Other restrictions may be made on the basis of the number of days stayed within a shorter period, such as a month. If your boyfriend's staying overnight 4+ times a week, that would violate pretty much every lease I've seen that contained such restrictions. However, if your lease or tenancy at will agreement did not specifically prohibit visitation, you may be able to successfully argue that your boyfriend can visit as often as you like. There are other potential wrinkles (possible violation of the state sanitary code, possible impact on subsidized housing) but those don't fit the general facts you've stated here. Good luck.