My longest period was when I was having complications in my pregnancy with my youngest son. We had to abstain for months, but it was all for the benefit of our baby's health.
Other than that, i don't know what you consider to be long periods weeks, months, years? We usually try at least on the weekends. He works full time, goes to school online full time, and we have two kids so it's hard to find the time so the weekend is usually when we are less stressed.