italian blend mix.....what should it look like?!

I bought the seeds & the seedlings are doing well, prob a very daft question, but I have no idea when I can start to harvest, the packet was not helpful either...at the moment they're about 2inches high, pretty much all looking the same. Is it weeks, months?  :oops:

thanks for the reply. Yes they're salad leaf...the packet just says "will crop over a long period if leaves are picked rather than whole plants cut" and to sow in drills in the spring or early summer....no other bit of info at all! was from organic cat too, all other packets of veg seeds been helpful so must be a one off.....or maybe most other people have bit more common sense than me!  
sowed them about 4 weeks ago, been in greenhouse, i'll try some in a week or 2 like u said