how long before between cockerels?

hi,

i currently have my legbar hens in with a welsummer cock, im going to put them with a legbar lad at the weekend but how long will i have to wait before i can hatch eggs that will be pure legbar?

is there a general rule or is it guess work? 

Apparantly they stay fertile for longer if you just remove the cockerel, rather than introduce another one... sometimes for up to 4 weeks!

As you are intorducing another boy I would wait at least three week before doing a test hatch