At least since they each declared that they're running for president.
Here's a full sampling of the tally of total combined Fox News Channel and Fox Business Network appearances for all the GOP candidates since their official campaign launches, per Politico:
1) Paul, 35 … 2) Huckabee, 31 … 3) Trump, 30 … 4) Perry, 24 … 5-6) Fiorina and Jindal, 20 each … 7) Cruz, 17 … 8) Santorum, 16 … 9) Rubio, 14 … 10-11) Carson and Graham, 12 each … 12-13) Kasich and Pataki, 11 each … 14) Christie, 7 … 15) Walker, 4 … 16) Bush, 3. [Politico]
The number of appearances doesn't quite track with candidates' running in the polls. Trump is the only candidate who appears both at the top of national polls and near the top of Fox News' list. The most recent Monmouth poll shows Fox's most frequent guest, Paul, at only 6 percent.