Just my perspective;
So I just started commuting last spring and the bike I purchased has a 5 year "unlimited tune-up" plan from the LBS. I've got to say, it was totally worth it (**more on that later)!
I am a fairly mechanically inclined person (I've rebuilt car engines), but bike mechanics just eludes me. My neighbor in the apartment below me used to be a bike mechanic, has shown me many things, I've read up on many things, but I just can't seem to get my mind around it for some reason. So, any time I feel an issue, rough shifting or similar, I just take it in to the LBS and about an hour later, problem solved! And it's free!
For me, it has been a very valuable resource to help keep my 15 mile a day commuter in tip-top shape. So far, in the past 8 months, I've taken it in 3 times. Just because. Hopefully some day soon,
I will get that eureka moment and it will all make sense, but until then, I'm happy that I have the plan and don't have to ride around frustrated. Most people I've known that don't stick with it usually stop due to frustration. I know from personal experience that said frustration often stems from easily solved mechanical issues.
So, here is my short answer to "how often should I get a tune up".....anytime you think you might need one.
**I actually bought my bike (2010 Globe Vienna 03 Disc) from Craigslist. The original buyer had purchased the service plan. I bought the bike from him only a month after he purchased it new (for 1/3 of retail). He gave me the original receipt and there is a service plan sticker on the frame. So far, the LBS has honored the plan without any questions.
Honestly, if I were the original buyer, I probably would have turned down the plan, but I'm sure glad he didn't! 