I am working on a campaign where my party will have a big adventure in Belkzen. What level is the average orc in Belkzen? I have read the orc guide but I still do not know what level an ordinary orc would be.
No average level was officially given
The Hold of Belkzen is a large piece of land, not a dungeon or city, and you can find creatures of all challenge ratings there, so the developers won't give you premade npc/monster sheets because you can find all sort of creatures there, from humans and halflings (like some shoanti) to dragons and liches (and i am dead serious). Even their capital city, Urgir, has about 1.150 humans, 290 elves, 275 half-elves and 881 others, and that is more human-ish population than a small town.
Which means that, even though the place has a lot of orcs, they are not the sole inhabitants there, and considering the size of the place, you can find orc farmers of 1/2 CR and orc chieftains of CR 15+. There is even a CR 16 lizard-like thing with a snake for tail roaming the Conquered Lands (southern area).
I suggest you get a copy of the Belzen, Hold of the Orc Hordes campaign setting book, you will get a ton of adventure ideas there with it, plus locations, geography, villages and cities, important NPCs, etc.