its usually a month and a half ish after the home one gets released. its a marketing ploy really coz if they release them both one after another straight away then the punters will only buy one. if they release them some time apart then they stand a chance of some thinking that they want a newer one than the one they already have
To be fair if they released them all together we'd moan about having to buy two shirts in the same month. Generally they're out shortly before the preseason friendlies so they can tie them in with ticket offers etc. Patience!