Detect magic has
Duration concentration, up to 1 min./level (D)
But nowhere is concentration clearly defined or the action required to concentrate on this spell.
There is concentration while casting a spell
To cast a spell, you must concentrate. If something interrupts your concentration while you’re casting, you must make a concentration check or lose the spell.
One way to consider this is that to benefit from detect magic you continue casting it for as long as you want to benefit from its effect. However since I know of no spell which provides an effect while casting it, clearly detect magic must be finished casting, otherwise it makes no sense.
The most logical way for this to work is its a standard action to concentrate on the spell, but its never actually said anywhere.
I have a similar question called Spells which require concentration such as detect magic, but it was never really answered to my satisfaction.