When was the last time you checked your dryer vent?

When was the last time you checked your dryer vent? It was a real eye-opener for me how quickly it can become clogged with lint and dust. Besides the obvious problem of decreased drying efficiency, I learned that a clogged vent can cause a fire. Signs to look out for include long drying times and an unusually hot drum. Now I'm thinking about finding professionals to help with this. How often do you check your vents? Should you trust professionals or can you do it yourself?