[Imp]

As far as I know there is no rough guide it would depend on the situation at the moment & also very much the country & even time period, Brits evacuating France for example.
Could be days or months esp if your talking say German vehicles in which case the replacement could well be anything similar or nothing at all.
On the Russian front German tank formations regulary operated at about 2/3rds to 3/4 strength.
Russia uses an entirely diffrent doctrine you would be lucky to get a trickle of replacements. They fight till the unit is at 1/3rd strength then disband it as a general rule.
So taking Russia generally no replacements but of course when it was defending its citys would often get a daily resupply. Tanks roling straight out of factorys into combat & conscript troops with a couple of days training doing there bit to save their familys.
So no hard & fast rule in my opinion.