Life is complicated. Some men love two women. Some men don't love their wives any more, but the marriage is companionable and he doesn't want to leave the kids. Some men are just dogs. Some men find they have different needs met by two different women, but they care about both in different ways. Perhaps, as AS said, men are creatures of habit. But I would say that the answer to "a long time", is that the man likes and cares about the mistress. If all he wanted was sex on the side, that is easy enough to find, and he wouldn't have to remain involved.

A woman who is the mistress of a married man needs to be aware of what the relationship actually is--the man may care about you and even love you, but his primary loyalty is to his wife.Because it turns into something much more than sex. Long term affairs typically are love affairs where the cheater is in a position where they feel they can’t leave the marriage despite the love they have for the mistress. They often feel trapped by fears and obligation and most of the motivation to stay is usually because of kids and finances. They find their personal and emotional fulfillment in the affair and their familial fulfillment with the kids at home.
I have been on both sides. I have been a wife of 10 years and a mistress of 6 (two separate relationships). I can tell you a man does in fact love and value his mistress. What a lot of people miss is that some marriages don’t work out and weren’t working before the mistress started dating the husband. I can say I’ve received the same or better treatment than that of the wife. In my case, he was married for 7 years and with me for 6 years. At the 6 year mark we opened a business together, and bought a home together. He left his wife. Today we are amazing. He has always been in love with me from day one and in my opinion never loved his wife like he loved me. Again, I think we were more compatible. He lied to me for about a year that he wasn’t married - until one day his wife contacted me. Even then, he told me he wasn’t and that she was crazy. I was hurt. I tried to leave but he wouldn’t let me. Years went by and he told me he wasn’t willing to leave and was willing to risk his marriage for me. I never gave him an ultimatum. I never brought up his marriage. I just enjoyed the time we spent. By year 4 I think that’s when things started changing. By year 5 we were discussing our future and plans and by year 6 he left her. He bought me a ring and we are engaged. We are crazy in love and our chemistry has never changed. As soon as we started the affair his family and friends knew. I was never in the dark. His wife knew and pretended I wasn’t there. She constantly told me that he was using me and I’d always laugh because I knew the truth. I don’t think she was the one he ever wanted. I think he married her for ‘papers’ so the marriage was never real, although she was convinced it was. She isn’t as attractive nor educated as I am. She has no assets. I have a couple of properties. I think he began falling for me based on what I had or what I could provide. Through the years he started his own business which took off and he Began supporting me. We’ve always been madly in love like two teenagers. We click. We have the same interests. We communicate well. He definitely loves me. It’s not lust. We’ve spent years together. I never asked him to leave her. He did on his own. Throughout the years he begged me to have children. I have a daughter who is an adult so I didn’t. A man can love his mistress. My father also married his mistress. He has been with Her for 20 years. The exact same time he was married to my mother. My Fiancé has never called me his mistress. In fact, he introduces me to everyone as his wife. It has been that way since day 1. We go out in public and do activities just like anyone else. We take trips together and we go to events together. He’s definitely in love with me and never hides his affection or love for me. He buys me anything I want. We even got marring tattoos while he was married. I guess I’m one of the rare cases of which a man loves his mistress so much that he leaves his wife. There are times I feel that he will never leave no matter what I say or do. Yes, a man does love his mistress!! I’m sorry wives out there - that is the frank truth.