That's a great question and is 100% market-specific. The only way you can know is to try and "pre-sell" the ad space. But that's normally hard as few advertisers understand or can envision a sign that does not exist. So I would build the sign that is 7 miles out and use that as your gauge to the value of the 12 and 17. I use to call this process using a "weather vane" sign -- a sign that is used to judge the demand in a market so that you know when and if you should build more. At 7 miles out, it stands to reason that, even if the market is initially weak, development will push "out" until your sign has many more potential players in the future.
My best "weather vane" story is the sign I build in the middle of nowhere on I-20 in Terrell, Texas. I was able to hardscrablle rent the sign to local advertisers (truck stops, etc.) but the initial demand was very weak. I was just able to cover the note. Then I got a strange call from someone wanting to rent the sign for a "mystery project" that I was able to find out was a new outlet mall that was about to be built. On the strength of that information, I ran out and secured every ground lease up and down the highway. When the mall was publicly announced, I already had the ground lease and permit on every sign location on I-20 in the area. People wanted to know my secret: how did I know that the area was about to "explode". The secret was that one "weather vane" sign. And the rents in that market shot from $250 per month to $1,200 per month within 24 months.
Another observation is that many advertisers are renting signs farther back from exits these days, because they have the exits covered with the standard informational signs that exist at most highway exits through Lamar's "logotype" sign program. So they are more focused on selling the customer on why they should exit. 7 miles back gives them about 5 minutes to discuss eating, lodging, etc. so that's a pretty good distance. In the old days, the advertiser only rented a sign right at the exit, because that was critical. With those signs that now show all the businesses at each exit, they can instead try to sell people farther back.