I've heard from people that you need to change up your workout every 6-8 weeks, but that's not my experience. I was adding weight to the bar every workout for three months before my injury and felt that I had probably another three months of gains left before I would plateau.
I do think that you should switch programs after you stop making gains for a while, just haven't gotten to that point yet.
I totally understand about taking years off. I lifted in college stopped after I graduated for about 5 years too. It's hard to get back into it. There's always an excuse not to go.
For myself, I I had to set a date and ease back into it . That first workout sucked and I was sore as hell for the next two days, but after a while you get into a routine and you start seeing gains and it helps keep you going.