The question each January, as Fox readies the premiere of its mega-hit reality show “American Idol,” is whether “Idol’s” hold on viewers will finally begin to soften.

It’s been the No. 1 show among adults 18-49 for the past three seasons and has yet to backslide in that time, pushing Fox to first place in the demographic the past two years.

In fact, all that success has resulted in a rather odd conundrum for Fox. Coming off the show’s highest-rated season ever, the network may not want it to grow any more, or advertisers will be priced right out of commercial spots.

“At this point, I don’t think Fox wants ‘Idol’ to get any bigger,” says one veteran media researcher. “Otherwise, they'll have a hard time finding advertising [because of the expense].”

This year a 30-second spot on the show will average more than $600,000, according to Ad Age, meaning most advertisers have already been priced out of TV’s most expensive show. What are advertisers getting for that money?

On the day of “Idol’s” sixth-season premiere, here’s a quick look at what media buyers and planners need to know about “Idol.”

“Idol” finished the past two seasons No. 1 in both adults 18-49 and total viewers. Last season was its best ever, averaging better than 30 million total viewers for the performance and results shows and up 14 percent over the previous year.

The show’s premiere and season finale were the two most-watched regularly scheduled broadcast programs of the season, averaging 35.5 million and 36.4 million total viewers, respectively. Both were series records, and up quite a bit from the 22.7 million who watched Kelly Clarkson win season one in September 2002.

Fox charged a reported $1.3 million per 30-second ad for May’s “Idol” finale, more than CBS got for the “Everybody Loves Raymond” series-ender in 2005.

It’s the priciest regular-season commercial buy by a long shot. ABC’s “Desperate Housewives” is No. 2 at a reported $394,000 per spot.

“Idol” is also one of the top shows in product placement. “Idol” had 446 minutes of product placement last year, up 12 percent over the previous year’s 400 minutes, according to Nielsen. 

“Idol” reaches a desirable audience. According to Magna Global, the median household income for “Idol” viewers is $60,000, or $10,000 less than last year’s highest-skewing show, “The West Wing.”

What’s more, about two thirds of “Idol” viewers live in households with three or more people, meaning the show draws a large family audience.

The median age of “Idol” viewers has crept up to 39 for Tuesday’s competition show and 40 for Wednesday’s results show, according to Magna. That’s four and seven years older than when the show started, but it’s a desirable graying. It’s helped attract more upscale advertisers to the show.

Viewers are downright rabid in their devotion to the show. The past two seasons, more than 500 million votes have been cast in the weekly voting that determines who stays and who goes.

Several “Idol” singers have released platinum albums, including winners Kelly Clarkson and Carrie Underwood and season two runner-up Clay Aiken.

A 2006 Lifetime movie about season three winner Fantasia Barrino was the network’s most-watched program of the year and ranked No. 40 on all of basic cable with 6.8 million total viewers.

Fox used “Idol” to lift Tuesday lead-out “House” into its No. 2 show, sticking the promising but struggling rookie program behind “Idol” for the first time in January 2005. Since then, “House” has risen to a top-five program among adults 18-49 even without the “Idol” lead-in and could well finish the season the No. 3 program in that demo, behind both “Idol” editions.

But the network has not been able to find a similarly potent combination Wednesdays, where a stream of reality and comedies have aired out of “Idol.” This year Fox has pledged to lay off the gimmicky reality shows in the hopes of growing a promising sitcom into a strong lead-out.

“I think it’s a mistake to constantly blow up our schedule,” Fox scheduling guru Preston Beckman told Media Life in November. “You get a lot of false positives that way, putting new shows behind big shows, but you don’t know if [the big ratings] are for the show or the time period.”