Really, half a second for A4s, but the Clan ones have 2.5 secs, seemed faster last time I used the Clan ones, and I was getting locked on by an MKII the other night really quick.
Also, it says an ELRM10 will lock in 0.1 seconds, so basically instantaneous, strange. Oh well, it's not like the devs will actually make Arrows difficult to use. C'est la vie.