Not finishing

So my husband and I have sex 1-2 times per week, but there is stretches of time where I don’t finish, like 2 weeks, then I will, finish, then another week or so goes by where I don’t again. I try to talk to him and tell him what I like, and that girls are like crockpots, it takes us longer to get aroused, but he just seems frustrated, and it almost seems like he doesn’t care. What tips can I give him? Or is this just normal not to finish every time?