I need to pour part of a 6" x 2' x 12' column between two overhead doors 10' x 12'. But I can only pour 8' up on the other sides of the doors, because of a cast in place floor system that goes in at the 8' level. Ideally I would pour the height of the doors and lintels all at the same time. Second choice would be to pour the column to the top of the opening, bottom of the lintel, leaving my rebar sticking two feet out the top.

But what I am wondering, is it okay to pour just 8' of the 12' column, leaving rebar sticking out six feet more?

If the reinforcing steel is extended into the next pour and laps the nexts pour's reinforcing enough to develop the reinforcing, then a cold joint is acceptable.  The old rule of thumb was a 48 bar diameter lap,; however, ACI now has specific laps for each size bar which may exceed 48 bar diamters.   

Jon

Personnaly I'd go all the way up and put in pockets for the floor joists, assuming it's hambro. If not hambro and a structural slab I'd look to put in a brickledge, if possible with the engeneers. My concern would be to keep that small column in line and same height for later on.
What rebar are you putting in for that column? If you don't mind me asking!
Good luck! Seems like a head scracher.

I am going up only 8' elsewhere because of the BuildDeck cast in place floor system, this will be tied into the walls without a brick-ledge. But in the area of the 12' high OH doors, it is open to a 17' ceiling. Horizontal #4 every 16" and four vertical #4 in the column.

Thanks for clarifications.
What I'd do is pour over the OH doors and insert appropriate lenght dowels horizontally at end wall for proper connection later on in the build.

I would go at least to the top of the doorway to allow the lintel pour to go 2 ft beyond the door opening. Depending on roof load etc. # 4 seams a little lite.

I think I will try to get the column to 12', the top of the opening, bottom of the lintel. Then the lintel will sit on it and be 5' deep.

ICF construction, you should probably run it by an engineer. That said you might take and idea or two to the engineer for review. Couple of ideas:

1) you could elimimate the foam entirely on the column -- e.g. make it 11"x2' -- although this is overkill since you have 2'. I'm putting it out there because sometimes you don't have the 2'. You could also eliminate the foam from only one side.
2) use #5 or #6 for your column (verticals). If your column were smaller it would also be a good idea to run horizontal stirrups around the four #6s.
3) engineer your lintel over the openings as if the column did not exist at all. E.g. like a 22' opening.
3) Assuming Builddeck is like Insuldeck, you could brace extensively and pour some or even all of your decking along with the wall (I don't like this one for a number of reasons).
4) Perhaps BETTER -- leave the column and this lintel (garage?) out of your wall pour and pour the column and lintel on the same day as you pour your deck. This will still require some clever bracing but seems doable easily enough. Regards.