Military leave in World War II

So I'm currently reading Tony Judt's Postwar and he mentions that, during World War II, there was no leave policy in the Soviet Army and that many soldiers fought continuously for years.

I'm curious if there were any exceptions to this (and if so, what where they?) and also how other countries dealt with military leave. Did the US, UK or Germany for that matter have leave policies for their troops?

Also, on a more general note, when and where did the practice of allowing soldiers periods of leave start?

Thanks!