How long.........

.......any ideas? (Bump)

Hmm, it all depends on how nasty your other hens are and how big the pen is, if they can go off and hide etc. they will kill them if they start pecking them. I don't let mine mix until they are a similar size to the adult birds.....I know you didn't want to hear that I section off the run and let them have their growers feed in peace. Though I think Nicos lets them mix much earlier?

Oh dear Snadger, did nobody answer you ....???
Poor chicks, and yes they can move pretty darned quick ......

Have they not settled in with the 'old girls' yet .....?
I know it's not the same, but when I added a cockerel to my girls - they were downright nasty ~ talk about henpecked ...!
He was a right big girls' blouse - and would run off to avoid them, and wouldn't come out of the 'house' in the morning until they let him ......
But they never properly 'attacked' him and after a week or so the cockerel (Jerome) was feeling his feathery feet and was taking care of 'his girls' !!

Probably not much help, though...

When the birds are a similar size they can cope with being henpecked...little chicks haven't develiped their feathers yet - if they get a spot of blood on them the birds won't stop until it's too late.

If the chicks have somewhere to hide which is too small for the other birds, you could try and mix them now...just stay with them.
Mommy should defend them big style ....or take them to somewhere 'safe'
A couple of retreat options are worth considering.

Play it by ear...you'll soon get a feeling for how protective mommy is.

One of my moms was absolutely brill defending her chicks ....

My bantam defended her chicks against all comers.
I used to put them out in a guinea pig run when they were very tiny while the others were free ranging so they all got a bit used to each other.
Then let them all free range together as the chicks entered their 'scruffy looking git' phase...but still seperated them in the evenings so the chicks got their growers pellets before bedtime.
Then finally put them all in together about ten days later....while mum still recognised them as hers.

The thing is, if yours aren't free rangers, I'm not sure the same rules apply.
I think perhaps the natural cover plants etc, the preoccupation with bug hunting and the shared threat of attack from above (sparrowhawks and so on) made it less likely for the flock to set upon the chicks.

Whatever, you must integrate them before mum forgets they are her chicks as she is their only defense.
Also, watch for those birds low down in the pecking order. I find they are for more likely to bully chicks...the guv'nors tend only to give the odd peck in passing but those docile, lowly girls you've always felt a bit sorry for?
They can be brutal and vicious.