Planting sweetcorn out

I am sure this topic has already been raised and answered but a search hasn't answered my question! 

Someone on here suggested a couple of weeks ago that I fill in a newly acquired space in my veggie plot with sweetcorn, so I sowed some in newspaper pots (two to a pot), and to my surprise the majority have germinated. They are now about 2 inches high and look like needles. My question is: at what point should I plant them out in the garden? The danger of frost is now past but do they need to be a certain height, or do I plant them when the roots try to escape or what? Help!

Thankyou - they need a couple more weeks yet by the looks of it. Presumably I have to remove the weaker seedling (or at least one out of the two as most of them look pretty healthy!)

Start hardening off too - putting out during the day, bringing in at night. They'll only be about a week or 10 days before they are ready to plant out if they are two inches already ...